`LEN` is incorrect.

[python-processing-unit]

According to the specification section 9.1.3:

• INT: LEN(INT|STR: a1, ..., INT|STR: aN) = MUST return the number of supplied arguments. Passing a TNS or any other unsupported type MUST raise a runtime error.

In the implementation, however, LEN behaves as a string/tensor-length builtin: it inspects the first argument and returns strlen() for STR or the first-dimension length for TNS, rejecting other types.