code: public boolean removeChild(Concept concept) {
          if (concept.parent != this) {
              throw new IllegalArgumentException("Concept to remove is not a child");
          }
  
          for (int i = 0; i < children.size(); i++) {
              if (children.get(i) == concept) {
                  children.remove(i);
                  concept.parent = null;
                  return true;
              }
          }
  
          return false;
      }
docs: Remove a child to this node