public Add(Expr var1, Expr var2) { _eVar1 = var1; _eVar2 = var2; }
public Mul(Expr var1, Expr var2) { _eVar1 = var1; _eVar2 = var2; }
public Sub(Expr var1, Expr var2) { _eVar1 = var1; _eVar2 = var2; }