Apply a visitor to a CST document or item.

Walks through the tree (depth-first) starting from the root, calling a visitor function with two arguments when entering each item:

• item: The current item, which included the following members:
  • start: SourceToken[] – Source tokens before the key or value, possibly including its anchor or tag.
  • key?: Token | null – Set for pair values. May then be null, if the key before the : separator is empty.
  • sep?: SourceToken[] – Source tokens between the key and the value, which should include the : map value indicator if value is set.
  • value?: Token – The value of a sequence item, or of a map pair.
• path: The steps from the root to the current node, as an array of ['key' | 'value', number] tuples.

The return value of the visitor may be used to control the traversal:

• undefined (default): Do nothing and continue
• visit.SKIP: Do not visit the children of this token, continue with next sibling
• visit.BREAK: Terminate traversal completely
• visit.REMOVE: Remove the current item, then continue with the next one
• number: Set the index of the next step. This is useful especially if the index of the current token has changed.
• function: Define the next visitor for this item. After the original visitor is called on item entry, next visitors are called after handling a non-empty key and when exiting the item.