在用syn做合成控制法的时候，报错expression too long，r（130）要怎么解决？没太懂这个错误的意思。
error . . . . . . . . . . . . . . . . . . . . . . . .  Return code 130
        expression too long
        too many SUMs
        In the first case, you specified an expression that is too long
        for Stata to process -- the expression contains more than 249
        pairs of nested parentheses or more than 800 dyadic operators.
        Break the expression into smaller parts.  In the second case, the
        expression contains more than 5 sum() functions.  This expression,
        too, will have to be broken into smaller parts.