code: public void end(Mapping mapping) {
          if (mapping == null) {
              throw new IllegalArgumentException("Method argument mapping must not be null");
          }
          OngoingMappings ongoingMappings = this.ongoingMappings.get();
          if (ongoingMappings != null) {
              ongoingMappings.remove(mapping);
              if (ongoingMappings.isEmpty()) {
                  this.ongoingMappings.remove();
              }
          }
      }
docs: Ends a mapping that was {@link #begin(Mapping) begun}.
  
  @param mapping must not be <code>null</code>.