code: public boolean isFlat() {
      boolean flat = true;
      for (int ci = 0; ci < this.parts.size(); ci++) {
        flat &= this.parts.get(ci).isPosTag();
      }
      return flat;
    }
docs: Returns true if this constituent contains no sub-constituents.
  
  @return true if this constituent contains no sub-constituents; false
  otherwise.