code: public Concept lowestCommonAncestor(String v, String w) {
          Concept vnode = getConcept(v);
          Concept wnode = getConcept(w);
  
          return lowestCommonAncestor(vnode, wnode);
      }
docs: Returns the lowest common ancestor (LCA) of concepts v and w. The lowest
  common ancestor is defined between two nodes v and w as the lowest node
  that has both v and w as descendants (where we allow a node to be a
  descendant of itself).