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Let start with an example. Suppose that we want to express information on the number feature of sentence phrase: If a sentences phrase consists of a noun phrase and a verb phrase then all of them should have the same number feature, either singular or plural. This can be expressed by the following two rules:
s(nb:pl) -> np(nb:pl) vp(nb:pl)
s(nb:sg) -> np(nb:sg) vp(nb:sg) Note that the categories are generalized to feature trees. When considering rules of this format, a parse tree for a sentence like the men run might look as follows.
n(cat:s(nb:pl)
n(cat:np(nb:pl)
n(cat:det(nb:pl)
leave(the))
n(cat:n(nb:pl)
leave(men)))
n(cat:vp(nb:pl)
n(cat:v(nb(pl))
leave(run))))Each inner node carries a feature tree for the category and has access to the child nodes. Each leave is labeled by a phoneme.
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