private void check(@Nullable GrExpression lhs) {
      if (!(lhs instanceof GrReferenceExpression)) {
        return;
      }
      final PsiElement referent = ((PsiReference) lhs).resolve();
      if (referent == null) {
        return;
      }
      if (!(referent instanceof GrParameter)) {
        return;
      }
      if (referent.getParent() instanceof GrForClause) {
        return;
      }
      registerError(lhs);
    }
private void check(@Nullable GrExpression lhs)