function link(heap, y, x) {
  removeFromRoot(heap, y);
  y.left = y;
  y.right = y;
  mergeWithChild(x, y);
  x.degree++;
  y.parent = x;
}
Function used to link one node to another in the root list.

@param {FibonacciHeap} heap - Target heap.
@param {Node}          y - Y node.
@param {Node}          x - X node.