datatemplate processing algorithm
input:
- datatemplate element t
- data tree rooted at node d
- element for which processing is happening, e
. do the magic to save the contents of e.
. create a list with form previousNodes[Node, namespace:string, tagName:string, templateIdentifier:string].
. for each node N with N.tagName, N.dataNode, N.templateIdentifier:
    add N to previousNodes[N.dataNode, N.namespace, N.tagName, N.templateIdentifier]
. remove all nodes from e.
. run processDataNode(d, '', e)

processDataNode(dataNode, mode, destination):
  . find the first matching rule for dataNode with mode mode.
    if no rules match, assume a template that just consists of a <nest/> element.
  . for each node templateNode that is a top-level child of this condition's template:
      run processTemplateNode(dataNode, templateNode, destination)

processTemplateNode(dataNode, templateNode, destination)
  . if templateNode is a 'nest' element:
      . let mode be the mode on the element.
      . for each child node childNode of dataNode that matches the filter on the element:
          . run processDataNode(childNode, mode, destination)
      . return
  . search previousNodes[] for [dataNode, templateNode.namespace, templateNode.tagName, templateNode.templateIdentifier]
    if you have a match previousNode:
      . remove previousNode from the previousNodes[] list.
      . remove all the children of previousNode.
      . move previousNode into destination.
      . for each attribute A on previousNode:
          . if templateNode has that attribute: update the previousNode[A] to match templateNode[A]
          . if templateNode doesn't have that attribute: remove previousNode[A]
      . for each attribute A on templateNode:
          . if previousNode doesn't have that attribute: add previousNode[A] to match templateNode[A]
      . let newNode be previousNode
    if you don't have a match previousNode:
      . shallow clone templateNode to cloneNode
      . set cloneNode.dataNode to dataNode
      . add cloneNode to destination.
      . let newNode be cloneNode
  . if newNode is an element or text node, perform attribute or text expansion on newNode.
  . for each node childTemplateNode that is a child of templateNode:
      . run processTemplateNode(dataNode, childTemplateNode, destination)