docs: public List<String> getSemanticHeads() {
      int headSemanticVariable = syntax.getHeadVariable();
      int[] allSemanticVariables = getSemanticVariables();
      for (int i = 0; i < allSemanticVariables.length; i++) {
        if (allSemanticVariables[i] == headSemanticVariable) {
          return Lists.newArrayList(variableAssignments.get(i));
        }
      }
      return Collections.emptyList();
    }