private Constraints.Constraint appendFields(
        Constraints.Constraint fields,
        String name,
        Constraints.Constraint setOfFields) {
      return !setOfFields.isNil() ? fields.equal(name, false, setOfFields) : fields;
    }
Returns chained criteria handle used to "OR" new constraint set to form logical DNF.
@return disjunction separated criteria handle
mongo/src/org/immutables/mongo/repository/Repositories.java