code: public String equivalence(final Namespace sourceNamespace,
              final String sourceValue, final Namespace targetNamespace)
              throws EquivalencerException {
  
          if (sourceNamespace == null) {
              throw new InvalidArgument("sourceNamespace", sourceNamespace);
          }
  
          if (noLength(sourceValue)) {
              throw new InvalidArgument("sourceValue", sourceValue);
          }
  
          if (targetNamespace == null) {
              throw new InvalidArgument("targetNamespace", targetNamespace);
          }
  
          loadEquivalencingEngine();
  
          // convert namespace and value to parameter (common model).
          final Parameter sp = new Parameter(sourceNamespace, sourceValue);
  
          try {
              Parameter parameter = this.paramEquivalencer.findEquivalence(sp,
                      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, sp.toBELShortForm());
              throw new EquivalencerException(msg, e);
          }
      }
docs: Finds a parameter, equivalent to the <tt>sourceParameter</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 sourceNamespace {@link Namespace}, the namespace of the source
  parameter
  @param sourceValue {@link String}, the source parameter value
  @param targetNamespace {@link Namespace}, the namespace of the equivalent
  parameter to find
  @return the equivalent parameter found in <tt>targetNamespace</tt>, or
  null if no equivalent parameter is found in that namespace
  @throw InvalidArgument Thrown if <tt>sourceNamespace</tt>,
  <tt>sourceParameter</tt>, or <tt>targetNamespace</tt> is null
  @throws EquivalencerException Thrown if an exception occurs finding the
  equivalent parameter