code: public MustacheEngineBuilder registerHelper(String name, Helper helper,
              boolean overwrite) {
          checkArgumentsNotNull(name, helper);
          checkNotBuilt();
          if (!overwrite && helpers.containsKey(name)) {
              throw new IllegalArgumentException(
                      "A helper with this name is already registered: " + name);
          }
          helpers.put(name, helper);
          return this;
      }
docs: Each helper must be registered with a unique name. If there is a helper
  registered with the same name and the param <code>overwrite</code> is
  <code>true</code> the previous instance is replaced, otherwise an
  {@link IllegalArgumentException} is thrown.
  
  @param name
  @param helper
  @param overwrite
  @return self