def prop_symbols(x):
    "Return a list of all propositional symbols in x."
    if not isinstance(x, Expr):
        return []
    elif is_prop_symbol(x.op):
        return [x]
    else:
        return list(set(symbol for arg in x.args
                        for symbol in prop_symbols(arg)))
Return a list of all propositional symbols in x.