Function isWellFormedFormula

Checks whether a given propositional expression is a well-formed formula (WFF).

A WFF follows these rules:

1. Any single variable itself is a WFF.
2. Any WFF can be prefixed with "~" to form another WFF.
3. Any two WFFs can be combined with "&", "|", "=>", or "<=>" inside parentheses to form another WFF.

Parentheses are mandatory when joining two WFFs using binary operators.