code: public Enumeration<String> getAvailableVariants(String source, String target) {
          CaseInsensitiveString cisrc = new CaseInsensitiveString(source);
          CaseInsensitiveString citrg = new CaseInsensitiveString(target);
          Map<CaseInsensitiveString, List<CaseInsensitiveString>> targets = specDAG.get(cisrc);
          if (targets == null) {
              return new IDEnumeration(null);
          }
          List<CaseInsensitiveString> variants = targets.get(citrg);
          if (variants == null) {
              return new IDEnumeration(null);
          }
          return new IDEnumeration(Collections.enumeration(variants));
      }
docs: Returns an enumeration over visible variant names for the given
  source and target.
  
  @return An <code>Enumeration</code> over <code>String</code> objects