['a] extension =
is, as the name says, the extensible part of the
for an introduction into extensions.
method clone :
< clone : 'b; node : 'a; set_node : 'a -> unit; .. > as 'b
clone is called when the node is to be duplicated, and as a
followup action, also the extension must be cloned. This method
must return a deep copy of the extension. By convention, the
caller of this method must also invoke
set_node on the copied
extension to establish a new link to a main node.
method node :
node is the link from the extension to the main node.
Conventionally, this link is set by the node via
below after the extension has been created or cloned.
method set_node :
'a -> unit
set_node n sets the link to the main node to
n. The link is
node after that.