code: public ModelNodeFormBuilder requiresAtLeastOne(String... attributeName) {
          if (attributeName != null && attributeName.length != 0) {
              this.requiresAtLeastOne.addAll(asList(attributeName));
          }
          return this;
      }
docs: Adds a validator to the form, to require at least one of them to be filled.
  Requires use of createValidators(true)