Return true if the string is a mathematical symbol.

def is_symbol(string):
    """
    Return true if the string is a mathematical symbol.
    """
    return (
        is_int(string) or is_float(string) or
        is_constant(string) or is_unary(string) or
        is_binary(string) or
        (string == '(') or (string == ')')
    )