code: public LinkVar getLinkVar(int parent, int child) {
          if (parent == -1) {
              return rootVars[child];
          } else {
              return childVars[parent][child];
          }
      }
docs: Get the link var corresponding to the specified parent and child position.
  
  @param parent The parent word position, or -1 to indicate the wall node.
  @param child The child word position.
  @return The link variable.