Use alphanumeric identifiers or double-quoted strings for atomic propositions, and parentheses for grouping.
Identifiers cannot start with the letter of a prefix operator (F, G, or X): for instance GFa means G(F(a)). Use "GFa" if you really want to refer to GFa as a proposition.
Conversely, infix letter operators are not assumed to be operators if they are part of an identifier: aUb is an atomic proposition, unlike a U b and (a)U(b).