Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
A c-commands B iff B is a sister of A, or B is dominated by a sister of A.
This one is easily implemented:
c_commands(A,B,Tree) :- sisters(A,AncB,Tree), dominates_or_eq(AncB,B,Tree).
(39) The relation between a tree and the string of its leaves, sometimes called the yield relation, is a little bit
more tricky to define. I will present a definition here, but not discuss it any detail. (Maybe in discussion
sections…)
yield(Tree,L) :- yield(Tree,[],L).
yield(W/[], L, [W|L]).
yield(_/[T|Ts], L0, L) :- yields([T|Ts],L0,L).
yields([], L, L).
yields([T|Ts], L0, L) :- yields(Ts,L0,L1), yield(T,L1,L).
NB: Notice that this does not distinguish empty categories with no yield from terminal vocabulary with
no yield.
3.7 Tree grammars
(40) The rules we have been considering so far rewrite strings. But it is not hard to formulate rules that
rewrite trees. Suppose for example that we have a tree:
s
a b c
(41) Suppose that we want to expand the category s in this tree with a rule that could be schematically
expressed as follows, where X,Y,Z are variables standing in for any subtrees:
s
X Y Z ⇒
x
a X
s
y
b Y
z
c Z
(42) If we apply this rule to the particular tree we began with, we get the result:
x
a a
s
y
b b
We could apply the rule again to this tree, and so on.
52
z
c c