Higher-order functions for computing the sibling of an element

hof.js

function findElement(node, stepper) {
  var x
  return isRoot(node)?       stepper(node)
  :      x = stepper(node)?  x
  :      /* otherwise */     findElement(node.parentNode, stepper)
}

function findNextSibling(node) {
  return findElement(node, function(el) { return el.nextSibling })
}

function findPrevSibling(node) {
  return findElement(node, function(el) { return el.previousSibling })
}