code: function featureIsUnifiable(feature) {
          logger.debug('TypedFeatureStructure.unifiable: checking feature: ' + feature + fs2.features[feature]);
          if (fs1.features[feature]) {
            // If feature of fs2 is a back pointer we want it to overrule feature
            // of the fs1.
            if (newStackb.indexOf(fs2.features[feature].id) > -1) {
              fs1.addAuxFeature(feature, fs2.features[feature], signature);
            }
            // One of the features should be not a back pointer because
            // otherwise we are checking for unification multiple times
            if ((newStacka.indexOf(fs1.features[feature].id) === -1) ||
              (newStackb.indexOf(fs2.features[feature].id) === -1)) {
              var unifiable = fs1.features[feature].unifiable(fs2.features[feature], signature, newStacka, newStackb);
              logger.debug('TypedFeatureStructure.unifiable: checking feature: ' + feature +
                ' with result: ' + unifiable);
              return (unifiable);
            }
            else {
              return (true);
            }
          }
          else {
            fs1.addAuxFeature(feature, fs2.features[feature], signature);
            return (true);
          }
        }
docs: Checks if a feature of two features structures can be unified