void Back_Assign(out NBase res) {
  res = null; NBase left; NBase expr;
  Back_AssignLeft(out left);
  Expect(65);
  Expr(out expr);
  res = new NAssign((NIdentBase)left, expr);
 }
 void Assign_Expr(out NBase res)
 {
     res = null; NBase left = null; NBase expr = null;
     if (TokenExists(0, "="))
     {
         Expr(out left);
         Expect(6);
         Expr(out expr);
         res = new NAssign(left, expr);
     }
     else if (StartOf(1))
     {
         Expr(out res);
     }
     else
     {
         SynErr(75);
     }
 }