比如mplus中相应的语句是：

GROUPING IS TECH(0=LOW 1=HIGH);!分组，TECH为调节变量

MODEL LOW:

  Y ON X(a);

MODEL HIGH:

  Y ON X(b);

MODEL CONSTRAINT:

NEW(DIFF);     !定义新变量

  DIFF= a - b;  

如何用lavaan实现？