Add a new node to a list node's children. If the item occurs after the list (olist < oitem), it is added as the last child. If the item occurs before the list, it is added as the first child. The optional keep arg can be "true" or "false". If "true", it keeps the nodes between the list and the item as children of list. If "false", it excises all the intervening nodes.
# Add a capitalized word
# to the _caps list, discarding
# the whitespace between them.
_xNIL <- _caps _xWHITE _xCAP @@
_xNIL <- _xCAP _xWHITE _caps @@