code: public boolean atEquivalentLocation(EntityFeature that) {
          return getEntityFeatureOf() != null &&
                  getEntityFeatureOf().isEquivalent(that.getEntityFeatureOf()) &&
                  getFeatureLocation() != null &&
                  getFeatureLocation().isEquivalent(that.getFeatureLocation());
      }
docs: This method returns true if and only if two entity features are on the same known location on a known ER.
  Unknown location or ER on any one of the features results in a false.