Search a node's descendants for a match. Stop looking down when a node is a leaf, has more than one child, or has the BASE attribute set.
s is an alternate form for singlet.
The singlet and base methodology enables attributes and ambiguities to be layered directly into the parse tree nodes, as a "singlet chain." By using the singlet modifer, a rule can pick up the attribute or possibility that is relevant.
# Even if a prior pass has reduced "per"
to a preposition, e.g., _prep,
# this rule will still match the word "per".
_adj <- per [s] capita @@
tree, Phrase Element Modifiers.