M-Expressions
Alternative syntax for Lisp, instead of S-Expressions. Virtually never used, and even less often implemented.
(en.wikipedia.org) M-expression - Wikipedia website
ROAM_REFS: https://en.wikipedia.org/wiki/M-expression
In computer programming, M-expressions (or meta-expressions) were an early proposed syntax for the Lisp programming language, inspired by contemporary languages such as Fortran and ALGOL. The notation was never implemented into the language and, as such, it was never finalized.
M-expressions are a syntax for LISP code and provide function notation, syntax for a
condform and for embedded literal data (via S-expressions) into programs. Thus M-Expressions used S-Expressions for literal data. The syntax for S-Expressions ("The Data Language") and M-Expressions ("The Meta Language") is defined on pages 8 and 9 of the Lisp 1.5 manual.M-Expressions also had a corresponding S-Expression representation. Code was manually translated from M-Expressions to S-Expressions. The in M-expressions embedded literal data, then had to be quoted in S-Expressions.