M4_(computer_language)

Never could get anything non-trivial to work correctly in M4. The company I worked for had an in-house macro language that I wanted to supplant. M4 does the standard macro substitution stuff just fine, but the bespoke macro language had features that I could never get right in M4. IIRC the real PITA was expanding the text twice and trying not to stomp on the newlines.

A macro language has the smell of something based on first-order logic proofs and trying to get any nontrivial user-defined function working in a macro language requires the ability to understand first-order logic proofs. Not fun when working on a deadline 😉