docs: public Function function(final TokVariable varTok) throws Exception
      {
          final String token = varTok.getValue().toString();
          final Constructor<?> constructor = functionMap.get(token.toLowerCase());
          if (constructor == null)
              return null;
          try
          {
              return (Function) constructor.newInstance(new Object[] {
                      varTok
              });
          } catch (final Exception e)
          {
              throw new Exception("function construction", e);
          }
      }