docs: private Expression2 getPreUnaryLogicalForm() {
      if (isTerminal()) {
        Expression2 logicalForm = lexiconEntry.getCategory().getLogicalForm();
        return logicalForm;
      } else {
        Expression2 leftLogicalForm = left.getLogicalForm();
        Expression2 rightLogicalForm = right.getLogicalForm();
  
        return getCombinatorLogicalForm(leftLogicalForm, rightLogicalForm);
      }
    }