code: public MustacheEngineBuilder registerHelper(String name, Helper helper) {
          return registerHelper(name, helper, false);
      }
docs: Each helper must be registered with a unique name. If there are more
  helpers registered with the same name an {@link IllegalArgumentException}
  is thrown. Use {@link #registerHelper(String, Helper, boolean)} to
  overwrite the helper.
  
  @param name
  @param helper
  @return self