Interface WalkerVisitMethods

1. When the node is first reached, the function is called with stage value "before".
2. If the node has fields, each field will then be visited in order:
   1. Before each field visit begins, the function is again called, but with the stage value "beforeField" and a field value indicating the field number (0 for the first field).
   2. Each node in the field is then visited, recursively, in order.
   3. After the last node in the field is visited, the function is called with stage "afterField" and the field number.
3. Once all of the fields have been visited, the function is called a final time for the node with stage "after".

Example: Visitation order for ½ x

before fraction
  beforeField 0 of fraction
     before digit 1
     after digit 1
  afterField 0 of fraction
  beforeField 1 of fraction
     before digit 2
     after digit 2
  afterField 1 of fraction
after fraction
before variable x
after variable x

Before each call to the visitor function, the walker position is updated to match the position of the visited node. Note that regardless of stage, the position is always the same: if the position were a caret position, the caret would be just before the node.

The order of node visits is determined by the document structure when the visit began. If a node is moved while visiting, it will be visited in its original order as if it had not moved. If a new node is inserted while visiting, it will be skipped by the visitor as if it were not present. If a node is removed while visiting, no further visits to that node will be made. Thus, if a node is removed at the "before" stage, then no "after" stage will occur, no fields will be visited, and none of node's children will be visited at all.