class type ['a Pxp_document.node #Pxp_document.extension as 'a]
index = object
.. end
method add : string -> 'a Pxp_document.node -> unit
Add the passed node to the index. If there is already an ID with
the passed string value, the exception ID_not_unique should be
raised. (But the index is free also to accept several identical IDs.)
method find : string -> 'a Pxp_document.node
Finds the node with the passed ID value, or raises Not_found