= Solution
\b[True.] In the formation rules, commas occur only between arguments in a function application or an atomic predicate application. Each such argument list includes an opening parenthesis. Subsequent construction of larger terms or formulas retains those symbols.
Equivalently prove by simultaneous <structural induction> on terms and formulas that an expression containing no opening parenthesis contains no comma. Variables and constants have neither; a comma-bearing function or predicate application already has an opening parenthesis; equality, connectives, and quantifiers cannot introduce a comma outside their constituent expressions. Taking the contrapositive proves the claim.
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