code: public static ConceptId getInstance(String id, Metadata metadata) {
          List<Object> key = new ArrayList<>(1);
          key.add(id);
          ConceptId result = metadata.getFromConceptIdCache(key);
          if (result != null) {
              return result;
          } else {
              result = new SimpleConceptId(id, metadata);
              metadata.putInConceptIdCache(key, result);
              return result;
          }
      }
docs: Returns a concept propId with the given string identifier.
  
  @param id a concept identifier {@link String}. Cannot be
  <code>null</code>.
  @return a {@link PropDefConceptId}.