code: function isLeftEdgeOf(node, ancestor) {
    while (node && node !== ancestor) {
      if (position(node) !== 0) {
        return false;
      }
      node = node.parentNode;
    }
  
    return true;
  }
docs: returns whether node is left edge of ancestor or not.
  
  @param {Node} node
  @param {Node} ancestor
  @return {Boolean}