A valid function name in an svex expressions.
Syntactically, we allow most symbols to be used as function names.
However, our expression language is fixed: only a few certain pre-defined
function symbols like bitnot, concat, etc., are understood by
functions like svex-eval and user-defined functions are not supported.
See functions for details.
- Equivalence relation for fnsyms.
- Recognizer for valid fnsyms.
- Fixing function for fnsyms.