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