Layer additional attributes for the element in the parse tree, as "mini-reductions".
Use the names in the list to name nodes. Each node that matched current rule element will be layered. layers, attrs, and attr are alternate forms for layer.
layer can also be used as a suggested element modifier.
# This rule reduces "fly" to a verb and then a vg (or "verb
_xNIL <- to fly [layer=(_verb _vg)] @@
Suggested Element Modifiers, Phrase Element Modifiers