'expression ::= term { (\'+\'|\'-\') term }*'

def expr(self):

    exprval = self.term()
    while (self._accept('PLUS') or self._accept('MINUS')):
        op = self.tok.type
        right = self.term()
        if (op == 'PLUS'):
            exprval += right
        elif (op == 'MINUS'):
            exprval -= right
    return exprval
