docs: public int getParentExpressionIndex(int index) {
      if (index == 0) {
        return -1;
      } else {
        int[] parts = findSubexpression(index);
        int parentIndex = subexpressions.get(parts[0]).getParentExpressionIndex(parts[1]);
        
        if (parentIndex == -1) {
          // index refers to a child of this expression.
          return 0;
        } else {
          // Return the index into this expression of the chosen child,
          // plus the index into that expression.
          return parentIndex + parts[2]; 
        }
      }
    }