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)))
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)))
def prop_symbols ( 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.
Return a list of all propositional symbols in x .