<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _xorterm(lexer):
 """Return an xor term expresssion."""

    prodterm = _prodterm(lexer)
    xorterm_prime = _xorterm_prime(lexer)
    if xorterm_prime is None:
        return prodterm
    else:
        return ('xor', prodterm, xorterm_prime)