diff --git a/zia-lang.org/crate/src/generated/css_classes.rs b/zia-lang.org/crate/src/generated/css_classes.rs
index 874154f9..5db2f2ad 100644
--- a/zia-lang.org/crate/src/generated/css_classes.rs
+++ b/zia-lang.org/crate/src/generated/css_classes.rs
@@ -1,4 +1,3 @@
-
 // DO NOT EDIT THIS FILE - IT'S GENERATED, CHANGES WILL BE LOST!
 
 #[allow(non_snake_case, dead_code)]
@@ -1592,7 +1591,6 @@ pub struct CssClasses<'a> {
 }
 
 pub static C: CssClasses = CssClasses {
-
     /**
         width: 100%;
         max-width: 569px;    @media (min-width: 569px)
@@ -3459,7 +3457,8 @@ pub static C: CssClasses = CssClasses {
         --tw-bg-opacity: 1;
         background-color: rgba(255, 0, 0, var(--tw-bg-opacity));
     */
-    group_hover__bg_unmatched_parenthesis: "group-hover:bg-unmatched_parenthesis",
+    group_hover__bg_unmatched_parenthesis:
+        "group-hover:bg-unmatched_parenthesis",
 
     /**
         --tw-bg-opacity: 1;
@@ -3501,7 +3500,8 @@ pub static C: CssClasses = CssClasses {
         --tw-bg-opacity: 1;
         background-color: rgba(255, 0, 0, var(--tw-bg-opacity));
     */
-    focus_within__bg_unmatched_parenthesis: "focus-within:bg-unmatched_parenthesis",
+    focus_within__bg_unmatched_parenthesis:
+        "focus-within:bg-unmatched_parenthesis",
 
     /**
         --tw-bg-opacity: 1;
@@ -4069,7 +4069,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;
         color: rgba(255, 0, 0, var(--tw-text-opacity));
     */
-    group_hover__text_unmatched_parenthesis: "group-hover:text-unmatched_parenthesis",
+    group_hover__text_unmatched_parenthesis:
+        "group-hover:text-unmatched_parenthesis",
 
     /**
         --tw-text-opacity: 1;
@@ -4111,7 +4112,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;
         color: rgba(255, 0, 0, var(--tw-text-opacity));
     */
-    focus_within__text_unmatched_parenthesis: "focus-within:text-unmatched_parenthesis",
+    focus_within__text_unmatched_parenthesis:
+        "focus-within:text-unmatched_parenthesis",
 
     /**
         --tw-text-opacity: 1;
@@ -6370,7 +6372,8 @@ pub static C: CssClasses = CssClasses {
         --tw-bg-opacity: 1;    @media (min-width: 569px)
         background-color: rgba(255, 0, 0, var(--tw-bg-opacity));    @media (min-width: 569px)
     */
-    sm__group_hover__bg_unmatched_parenthesis: "sm:group-hover:bg-unmatched_parenthesis",
+    sm__group_hover__bg_unmatched_parenthesis:
+        "sm:group-hover:bg-unmatched_parenthesis",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 569px)
@@ -6388,19 +6391,22 @@ pub static C: CssClasses = CssClasses {
         --tw-bg-opacity: 1;    @media (min-width: 569px)
         background-color: rgba(255, 191, 0, var(--tw-bg-opacity));    @media (min-width: 569px)
     */
-    sm__focus_within__bg_variable_concept: "sm:focus-within:bg-variable_concept",
+    sm__focus_within__bg_variable_concept:
+        "sm:focus-within:bg-variable_concept",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 569px)
         background-color: rgba(191, 0, 255, var(--tw-bg-opacity));    @media (min-width: 569px)
     */
-    sm__focus_within__bg_abstract_concept: "sm:focus-within:bg-abstract_concept",
+    sm__focus_within__bg_abstract_concept:
+        "sm:focus-within:bg-abstract_concept",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 569px)
         background-color: rgba(64, 0, 255, var(--tw-bg-opacity));    @media (min-width: 569px)
     */
-    sm__focus_within__bg_concrete_concept: "sm:focus-within:bg-concrete_concept",
+    sm__focus_within__bg_concrete_concept:
+        "sm:focus-within:bg-concrete_concept",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 569px)
@@ -6412,7 +6418,8 @@ pub static C: CssClasses = CssClasses {
         --tw-bg-opacity: 1;    @media (min-width: 569px)
         background-color: rgba(255, 0, 0, var(--tw-bg-opacity));    @media (min-width: 569px)
     */
-    sm__focus_within__bg_unmatched_parenthesis: "sm:focus-within:bg-unmatched_parenthesis",
+    sm__focus_within__bg_unmatched_parenthesis:
+        "sm:focus-within:bg-unmatched_parenthesis",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 569px)
@@ -6956,19 +6963,22 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(255, 191, 0, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__group_hover__text_variable_concept: "sm:group-hover:text-variable_concept",
+    sm__group_hover__text_variable_concept:
+        "sm:group-hover:text-variable_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(191, 0, 255, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__group_hover__text_abstract_concept: "sm:group-hover:text-abstract_concept",
+    sm__group_hover__text_abstract_concept:
+        "sm:group-hover:text-abstract_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(64, 0, 255, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__group_hover__text_concrete_concept: "sm:group-hover:text-concrete_concept",
+    sm__group_hover__text_concrete_concept:
+        "sm:group-hover:text-concrete_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
@@ -6980,7 +6990,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(255, 0, 0, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__group_hover__text_unmatched_parenthesis: "sm:group-hover:text-unmatched_parenthesis",
+    sm__group_hover__text_unmatched_parenthesis:
+        "sm:group-hover:text-unmatched_parenthesis",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
@@ -6998,19 +7009,22 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(255, 191, 0, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__focus_within__text_variable_concept: "sm:focus-within:text-variable_concept",
+    sm__focus_within__text_variable_concept:
+        "sm:focus-within:text-variable_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(191, 0, 255, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__focus_within__text_abstract_concept: "sm:focus-within:text-abstract_concept",
+    sm__focus_within__text_abstract_concept:
+        "sm:focus-within:text-abstract_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(64, 0, 255, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__focus_within__text_concrete_concept: "sm:focus-within:text-concrete_concept",
+    sm__focus_within__text_concrete_concept:
+        "sm:focus-within:text-concrete_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
@@ -7022,7 +7036,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(255, 0, 0, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__focus_within__text_unmatched_parenthesis: "sm:focus-within:text-unmatched_parenthesis",
+    sm__focus_within__text_unmatched_parenthesis:
+        "sm:focus-within:text-unmatched_parenthesis",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
@@ -7064,7 +7079,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(255, 0, 0, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__hover__text_unmatched_parenthesis: "sm:hover:text-unmatched_parenthesis",
+    sm__hover__text_unmatched_parenthesis:
+        "sm:hover:text-unmatched_parenthesis",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 569px)
@@ -7106,7 +7122,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 569px)
         color: rgba(255, 0, 0, var(--tw-text-opacity));    @media (min-width: 569px)
     */
-    sm__focus__text_unmatched_parenthesis: "sm:focus:text-unmatched_parenthesis",
+    sm__focus__text_unmatched_parenthesis:
+        "sm:focus:text-unmatched_parenthesis",
 
     /**
         text-decoration: underline;    @media (min-width: 569px)
@@ -9261,7 +9278,8 @@ pub static C: CssClasses = CssClasses {
         --tw-bg-opacity: 1;    @media (min-width: 1025px)
         background-color: rgba(255, 0, 0, var(--tw-bg-opacity));    @media (min-width: 1025px)
     */
-    lg__group_hover__bg_unmatched_parenthesis: "lg:group-hover:bg-unmatched_parenthesis",
+    lg__group_hover__bg_unmatched_parenthesis:
+        "lg:group-hover:bg-unmatched_parenthesis",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 1025px)
@@ -9279,19 +9297,22 @@ pub static C: CssClasses = CssClasses {
         --tw-bg-opacity: 1;    @media (min-width: 1025px)
         background-color: rgba(255, 191, 0, var(--tw-bg-opacity));    @media (min-width: 1025px)
     */
-    lg__focus_within__bg_variable_concept: "lg:focus-within:bg-variable_concept",
+    lg__focus_within__bg_variable_concept:
+        "lg:focus-within:bg-variable_concept",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 1025px)
         background-color: rgba(191, 0, 255, var(--tw-bg-opacity));    @media (min-width: 1025px)
     */
-    lg__focus_within__bg_abstract_concept: "lg:focus-within:bg-abstract_concept",
+    lg__focus_within__bg_abstract_concept:
+        "lg:focus-within:bg-abstract_concept",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 1025px)
         background-color: rgba(64, 0, 255, var(--tw-bg-opacity));    @media (min-width: 1025px)
     */
-    lg__focus_within__bg_concrete_concept: "lg:focus-within:bg-concrete_concept",
+    lg__focus_within__bg_concrete_concept:
+        "lg:focus-within:bg-concrete_concept",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 1025px)
@@ -9303,7 +9324,8 @@ pub static C: CssClasses = CssClasses {
         --tw-bg-opacity: 1;    @media (min-width: 1025px)
         background-color: rgba(255, 0, 0, var(--tw-bg-opacity));    @media (min-width: 1025px)
     */
-    lg__focus_within__bg_unmatched_parenthesis: "lg:focus-within:bg-unmatched_parenthesis",
+    lg__focus_within__bg_unmatched_parenthesis:
+        "lg:focus-within:bg-unmatched_parenthesis",
 
     /**
         --tw-bg-opacity: 1;    @media (min-width: 1025px)
@@ -9847,19 +9869,22 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(255, 191, 0, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__group_hover__text_variable_concept: "lg:group-hover:text-variable_concept",
+    lg__group_hover__text_variable_concept:
+        "lg:group-hover:text-variable_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(191, 0, 255, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__group_hover__text_abstract_concept: "lg:group-hover:text-abstract_concept",
+    lg__group_hover__text_abstract_concept:
+        "lg:group-hover:text-abstract_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(64, 0, 255, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__group_hover__text_concrete_concept: "lg:group-hover:text-concrete_concept",
+    lg__group_hover__text_concrete_concept:
+        "lg:group-hover:text-concrete_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
@@ -9871,7 +9896,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(255, 0, 0, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__group_hover__text_unmatched_parenthesis: "lg:group-hover:text-unmatched_parenthesis",
+    lg__group_hover__text_unmatched_parenthesis:
+        "lg:group-hover:text-unmatched_parenthesis",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
@@ -9889,19 +9915,22 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(255, 191, 0, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__focus_within__text_variable_concept: "lg:focus-within:text-variable_concept",
+    lg__focus_within__text_variable_concept:
+        "lg:focus-within:text-variable_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(191, 0, 255, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__focus_within__text_abstract_concept: "lg:focus-within:text-abstract_concept",
+    lg__focus_within__text_abstract_concept:
+        "lg:focus-within:text-abstract_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(64, 0, 255, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__focus_within__text_concrete_concept: "lg:focus-within:text-concrete_concept",
+    lg__focus_within__text_concrete_concept:
+        "lg:focus-within:text-concrete_concept",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
@@ -9913,7 +9942,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(255, 0, 0, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__focus_within__text_unmatched_parenthesis: "lg:focus-within:text-unmatched_parenthesis",
+    lg__focus_within__text_unmatched_parenthesis:
+        "lg:focus-within:text-unmatched_parenthesis",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
@@ -9955,7 +9985,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(255, 0, 0, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__hover__text_unmatched_parenthesis: "lg:hover:text-unmatched_parenthesis",
+    lg__hover__text_unmatched_parenthesis:
+        "lg:hover:text-unmatched_parenthesis",
 
     /**
         --tw-text-opacity: 1;    @media (min-width: 1025px)
@@ -9997,7 +10028,8 @@ pub static C: CssClasses = CssClasses {
         --tw-text-opacity: 1;    @media (min-width: 1025px)
         color: rgba(255, 0, 0, var(--tw-text-opacity));    @media (min-width: 1025px)
     */
-    lg__focus__text_unmatched_parenthesis: "lg:focus:text-unmatched_parenthesis",
+    lg__focus__text_unmatched_parenthesis:
+        "lg:focus:text-unmatched_parenthesis",
 
     /**
         text-decoration: underline;    @media (min-width: 1025px)
diff --git a/zia-lang.org/crate/src/page/home/tutorials.rs b/zia-lang.org/crate/src/page/home/tutorials.rs
index 6ed53fbe..d185acd4 100644
--- a/zia-lang.org/crate/src/page/home/tutorials.rs
+++ b/zia-lang.org/crate/src/page/home/tutorials.rs
@@ -1,7 +1,7 @@
 #[cfg(test)]
 use std::time::{Duration, Instant};
 
-pub const TUTORIALS: (Tutorial<18>, Tutorial<9>, Tutorial<12>) = (
+pub const TUTORIALS: (Tutorial<18>, Tutorial<10>, Tutorial<12>) = (
     Tutorial {
         title: "Factorial",
         steps: [
@@ -154,6 +154,12 @@ pub const TUTORIALS: (Tutorial<18>, Tutorial<9>, Tutorial<12>) = (
                 #[cfg(test)]
                 expected_evaluation: "false"
             },
+            TutorialStep {
+                command: "let Alice is child of Bob",
+                explanation: "We won't accidentally define a parent as a child of their child",
+                #[cfg(test)]
+                expected_evaluation: "Alice is child of Bob already reduces to false"
+            },
             TutorialStep {
                 command: "let ((_y_ exists_such_that) (_x_ is parent of _y_) and (_y_ is parent of _z_)) => (_x_ is grandparent of _z_)",
                 explanation: "Define grandparent is terms of parent relationships",
diff --git a/zia/README.md b/zia/README.md
index c5cda61d..f78c7527 100755
--- a/zia/README.md
+++ b/zia/README.md
@@ -21,8 +21,8 @@ binary tree of height 2 with leaves `"ll"`, `"lr"`, `"rl"`, `"rr"` going from le
 The leaves of the tree can be any unicode string without spaces or parentheses. These symbols may
 be recognised by the intepreter as concepts or if not used to label new concepts.
 
-So far there are 10 built-in concepts. A new `Context` labels these with the symbols, `label_of`,
-`->`, `:=`, `let`, `true`, `false`, `assoc`, `right`, `left`, `precedes`, `>`, `=>` and
+So far there are 14 built-in concepts. A new `Context` labels these with the symbols, `label_of`,
+`->`, `:=`, `let`, `true`, `false`, `assoc`, `right`, `left`, `precedes`, `>`, `=>`, `forget` and
 `exists_such_that` but the concepts can be renamed (via the `:=` concept, as shown below) to
 different symbols for different languages or disciplines.
 
@@ -40,6 +40,7 @@ assert_eq!(context.execute("let a b -> c"), "");
 assert_eq!(context.execute("a b"), "c");
 
 // Change the rule so that concept "a b" instead reduces to concept "d"
+assert_eq!(context.execute("forget a b"), "");
 assert_eq!(context.execute("let a b -> d"), "");
 assert_eq!(context.execute("a b"), "d");
 
@@ -50,7 +51,7 @@ assert_eq!(context.execute("a b"), "a b");
 // Try to specify a rule that already exists
 assert_eq!(context.execute("let a b -> a b"), ZiaError::RedundantReduction{syntax: "a b".to_string()}.to_string());
 assert_eq!(context.execute("let a b -> c"), "");
-assert_eq!(context.execute("let a b -> c"), ZiaError::RedundantReduction{syntax: "a b".to_string()}.to_string());
+assert_eq!(context.execute("let a b -> c"), ZiaError::ExistingReduction{syntax_to_reduce: "a b".to_string(), existing_reduction: "c".to_string()}.to_string());
 
 // Relabel "label_of" to "표시"
 assert_eq!(context.execute("let 표시 := label_of"), "");
@@ -87,9 +88,6 @@ assert_eq!(context.execute("g"), "true");
 assert_eq!(context.execute("let h i j"), "");
 assert_eq!(context.execute("h i j"), "true");
 
-// Determine associativity of symbol
-assert_eq!(context.execute("assoc a"), "right");
-
 // Define patterns
 assert_eq!(context.execute("let _x_ or true -> true"), "");
 assert_eq!(context.execute("false or true"), "true");
diff --git a/zia/src/concepts/mod.rs b/zia/src/concepts/mod.rs
index ce6058cf..63576802 100755
--- a/zia/src/concepts/mod.rs
+++ b/zia/src/concepts/mod.rs
@@ -740,6 +740,7 @@ pub enum ConcreteConceptType {
     Implication,
     ExistsSuchThat,
     Precedes,
+    Forget,
 }
 
 impl<Id: Eq + Hash> From<ConcreteConceptType> for SpecificPart<Id> {
diff --git a/zia/src/context.rs b/zia/src/context.rs
index e65c559f..0f018b62 100755
--- a/zia/src/context.rs
+++ b/zia/src/context.rs
@@ -800,6 +800,7 @@ where
             ("=>", ConcreteConceptType::Implication),
             ("exists_such_that", ConcreteConceptType::ExistsSuchThat),
             ("precedes", ConcreteConceptType::Precedes),
+            ("forget", ConcreteConceptType::Forget),
         ];
         let maybe_inner_delta = self.delta.get_mut();
         let Some(delta) = maybe_inner_delta else {
@@ -956,6 +957,10 @@ where
                         maybe_ast.map(|ast| self.execute_reduction(right, &ast))
                     })
                     .map(|r| r.map(|()| String::new())),
+                ConcreteConceptType::Forget => right.get_concept().map(|c| {
+                    self.updater()?.delete_reduction(c, right.to_string())?;
+                    Ok(String::new())
+                }),
                 ConcreteConceptType::Label => Some(Ok("'".to_string()
                     + &right
                         .get_concept()
@@ -1159,6 +1164,18 @@ where
         } else if syntax.key() == normal_form.key() {
             self.try_removing_reduction(syntax)
         } else {
+            let context_search = self.context_search();
+            if let Some((existing_reduction, _)) =
+                context_search.reduce(syntax).or_else(|| {
+                    context_search.find_examples_of_inferred_reduction(syntax)
+                })
+            {
+                return Err(ZiaError::ExistingReduction {
+                    syntax_to_reduce: syntax.to_string(),
+                    existing_reduction: existing_reduction.to_string(),
+                });
+            }
+            drop(context_search);
             let maybe_inner_delta = self.delta.get_mut();
             let Some(delta) = maybe_inner_delta else {
                 return Err(ZiaError::MultiplePointersToDelta);
diff --git a/zia/src/context_search.rs b/zia/src/context_search.rs
index b6ad15b5..66613269 100644
--- a/zia/src/context_search.rs
+++ b/zia/src/context_search.rs
@@ -171,41 +171,6 @@ where
         })
     }
 
-    // If (operator right)  cannot by trying be reduced by other means, then it should reduce to default_concept
-    fn reduce_otherwise_default(
-        &self,
-        ast: &SharedSyntax<CCI, SR>,
-        left: &SharedSyntax<CCI, SR>,
-        right: &SharedSyntax<CCI, SR>,
-        operator_id: &S::ConceptId,
-        default_concept_id: &S::ConceptId,
-    ) -> ReductionResult<CCI, SR> {
-        let mut reduced_pair: ReductionResult<CCI, SR> = None;
-        let mut operator_composition_check = || {
-            let cache = SR::share(ReductionCache::<CCI, SR>::default());
-            let mut context_search = self.spawn(&cache, self.delta.clone());
-            context_search.syntax_evaluating.insert(ast.key());
-            reduced_pair = context_search.reduce_pair(left, right);
-            let operator_concept =
-                self.snap_shot.read_concept(self.delta.as_ref(), *operator_id);
-            let find = |c| {
-                operator_concept.find_as_hand_in_composition_with(c, Hand::Left)
-            };
-            reduced_pair.is_none()
-                && right.get_concept().and_then(find).is_none()
-        };
-        if self.syntax_evaluating.contains(&ast.key())
-            || operator_composition_check()
-        {
-            Some((
-                self.to_ast(default_concept_id),
-                ReductionReason::<CCI, SR>::default(*operator_id),
-            ))
-        } else {
-            reduced_pair
-        }
-    }
-
     /// Reduces the syntax by using the reduction rules of associated concepts.
     pub fn reduce(
         &self,
@@ -223,24 +188,7 @@ where
                 .and_then(|c| self.reduce_concept(&c))
                 .or_else(|| {
                     let (ref left, ref right) = ast.get_expansion()?;
-                    left.get_concept()
-                        .and_then(|lc| match self.concrete_type(&lc) {
-                            Some(ConcreteConceptType::Associativity) => {
-                                let default_concept_id = self
-                                    .concrete_concept_id(
-                                        ConcreteConceptType::Right,
-                                    )?;
-                                self.reduce_otherwise_default(
-                                    ast,
-                                    left,
-                                    right,
-                                    &lc,
-                                    &default_concept_id,
-                                )
-                            },
-                            _ => None,
-                        })
-                        .or_else(|| self.reduce_pair(left, right))
+                    self.reduce_pair(left, right)
                 });
             self.caches.insert_reduction(ast, &reduction_result);
             reduction_result
diff --git a/zia/src/errors.rs b/zia/src/errors.rs
index d57d6432..7a152b7e 100755
--- a/zia/src/errors.rs
+++ b/zia/src/errors.rs
@@ -107,4 +107,9 @@ pub enum ZiaError {
     CanOnlyQuantifyOverVariables,
     #[error("Multiple smart pointers to context delta")]
     MultiplePointersToDelta,
+    #[error("{syntax_to_reduce} already reduces to {existing_reduction}")]
+    ExistingReduction {
+        syntax_to_reduce: String,
+        existing_reduction: String,
+    },
 }
diff --git a/zia/src/lib.rs b/zia/src/lib.rs
index 58242614..efbddf6b 100755
--- a/zia/src/lib.rs
+++ b/zia/src/lib.rs
@@ -33,8 +33,8 @@
 //! The leaves of the tree can be any unicode string without spaces or parentheses. These symbols may
 //! be recognised by the intepreter as concepts or if not used to label new concepts.
 //!
-//! So far there are 10 built-in concepts. A new `Context` labels these with the symbols, `label_of`,
-//! `->`, `:=`, `let`, `true`, `false`, `assoc`, `right`, `left`, `precedes`, `>`, `=>` and
+//! So far there are 14 built-in concepts. A new `Context` labels these with the symbols, `label_of`,
+//! `->`, `:=`, `let`, `true`, `false`, `assoc`, `right`, `left`, `precedes`, `>`, `=>`, `forget` and
 //! `exists_such_that` but the concepts can be renamed (via the `:=` concept, as shown below) to
 //! different symbols for different languages or disciplines.
 //!
@@ -52,6 +52,7 @@
 //! assert_eq!(context.execute("a b"), "c");
 //!
 //! // Change the rule so that concept "a b" instead reduces to concept "d"
+//! assert_eq!(context.execute("forget a b"), "");
 //! assert_eq!(context.execute("let a b -> d"), "");
 //! assert_eq!(context.execute("a b"), "d");
 //!
@@ -62,7 +63,7 @@
 //! // Try to specify a rule that already exists
 //! assert_eq!(context.execute("let a b -> a b"), ZiaError::RedundantReduction{syntax: "a b".to_string()}.to_string());
 //! assert_eq!(context.execute("let a b -> c"), "");
-//! assert_eq!(context.execute("let a b -> c"), ZiaError::RedundantReduction{syntax: "a b".to_string()}.to_string());
+//! assert_eq!(context.execute("let a b -> c"), ZiaError::ExistingReduction{syntax_to_reduce: "a b".to_string(), existing_reduction: "c".to_string()}.to_string());
 //!
 //! // Relabel "label_of" to "표시"
 //! assert_eq!(context.execute("let 표시 := label_of"), "");
@@ -99,9 +100,6 @@
 //! assert_eq!(context.execute("let h i j"), "");
 //! assert_eq!(context.execute("h i j"), "true");
 //!
-//! // Determine associativity of symbol
-//! assert_eq!(context.execute("assoc a"), "right");
-//!
 //! // Define patterns
 //! assert_eq!(context.execute("let _x_ or true -> true"), "");
 //! assert_eq!(context.execute("false or true"), "true");
diff --git a/zia/tests/reductions.rs b/zia/tests/reductions.rs
index 8a8c8148..8ed3e017 100644
--- a/zia/tests/reductions.rs
+++ b/zia/tests/reductions.rs
@@ -81,6 +81,7 @@ fn broken_middle_chain() {
 fn change_reduction_rule() {
     let mut cont = NEW_CONTEXT.clone();
     assert_eq!(cont.execute("let a b -> c"), "");
+    assert_eq!(cont.execute("forget a b"), "");
     assert_eq!(cont.execute("let a b -> d"), "");
     assert_eq!(cont.execute("a b"), "d");
 }
@@ -90,6 +91,7 @@ fn leapfrog_reduction_rule() {
     let mut cont = NEW_CONTEXT.clone();
     assert_eq!(cont.execute("let a b -> c d"), "");
     assert_eq!(cont.execute("let c d -> e"), "");
+    assert_eq!(cont.execute("forget a b"), "");
     assert_eq!(cont.execute("let a b -> e"), "");
 }
 // It is redundant to specify a reduction rule that is already true
@@ -99,8 +101,9 @@ fn redundancy() {
     assert_eq!(cont.execute("let a b -> c"), "");
     assert_eq!(
         cont.execute("let a b -> c"),
-        ZiaError::RedundantReduction {
-            syntax: "a b".into()
+        ZiaError::ExistingReduction {
+            syntax_to_reduce: "a b".into(),
+            existing_reduction: "c".into()
         }
         .to_string()
     );