private @Nullable UnresolvableRenameProblem findProblem() {
    if (!isIdentifier(myInsertedName, myLanguage)) {
      return new IllegalIdentifierProblem();
    }
    UnresolvableCollisionUsageInfo collision = findCollision();
    if (collision != null) {
      return new Collision(collision);
    }
    return null;
  }
private @Nullable UnresolvableRenameProblem findProblem()