code: public String equivalence(SkinnyUUID sourceUUID, Namespace targetNamespace)
              throws EquivalencerException {
          if (sourceUUID == null) {
              throw new InvalidArgument("sourceUUID", sourceUUID);
          }
  
          if (targetNamespace == null) {
              throw new InvalidArgument("targetNamespace", targetNamespace);
          }
  
          loadEquivalencingEngine();
  
          try {
              Parameter parameter = this.paramEquivalencer.findEquivalence(
                      sourceUUID, targetNamespace);
              return parameter == null ? null : parameter.getValue();
          } catch (InvalidArgument e) {
              //TODO change exception when paramEquivalencer is changed
              return null;
          } catch (Exception e) {
              final String fmt = "Unable to find equivalences for '%s'";
              final String msg = format(fmt, sourceUUID.toString());
              throw new EquivalencerException(msg, e);
          }
      }
docs: Finds a parameter, equivalent to the <tt>sourceUUID</tt>, and found in
  the {@link Namespace} identified by <tt>targetNamespace</tt>.
  <p>
  Note: The first time any method in {@link Equivalencer} is called the
  equivalencing engine is instantiated, but only once. The equivalencing
  engine loads the BEL equivalence files associated with the framework and
  uses them do perform equivalencing.
  </p>
  
  @param sourceUUID
  @param targetNamespace
  @return the equivalent parameters or an empty map if no equivalent
  parameter is found
  @throws EquivalencerException Thrown if an exception occurs finding all
  equivalences