Combining source typst files that use conflicting show rules

[breezykermo]

A problem that Rheo introduce is how we should combine Typst files that present conflicting show rules. Consider, for example, two source files combined into a PDF:

a.typ:

#show: // use template A

b.typ:

#show: // use template B

rheo.toml:

[pdf.spine]
title = "conflicting show rules"
vertebrae = ["a.typ", "b.typ"]
merge = true

Because the show rules conflict, Rheo cannot meaningfully 'merge' these two files: it needs to either disregard some show rules, or present a compilation error.