Consider the following TAG in figure 4.21.
As an example of the encoding of such elementary trees in
(
),
consider the encoding of
in figure 4.22 and the
encoding of
in figure 4.23.
Consider what happens if we parse the sentence ``the very pretty girl
left today''. The first goal of the parser, is to find a tree of which the root
node has category s. In order to find such a goal, a non-chain-rule
is selected which has also s as its root node, and of which the string
is part of the input string.
The rule which is
selected is rule . The daughter of this rule is the
substitution node np. Therefore, the embedded parse goal is to
parse an np, with bag of words [the,very,pretty,girl,today]. Again, the first step of the parser
consists in the prediction of a non-chain-rule, of which the root has category
np, and of which the string is part of the input bag.
This time
is selected and we obtain another embedded parse goal: the parsing of
a d with bag [the,very,pretty,today]. The non-chain-rule
which is selected for this goal is
. As this tree does not
have any substitution nodes, we can immediately connect
to
the goal d. As no auxiliary trees apply to
, connection
is trivial, and we finish the embedded parse goal for d. We thus
continue parsing of an np, with head
, and of which the
substitution nodes are filled in, in the mean time. To connect this tree
upward to the np goal, the auxiliary rule
may be
applied. After the application of that auxiliary tree we obtain the
tree:
Again, this tree should be connected upward to the NP goal, with
input bag [very,today], and
another auxiliary tree can be applied: , giving tree:
Finally, this connects the input tree to the np goal, and this embedded parse is finished. Therefore, we continue the parsing of the s goal, with input bag [today], and of which the head is instantiated as:
The auxiliary rule
applies, giving the tree:
which we connect trivially to the goal, as there are no more words left in the input bag.
This example clearly shows how the parser first selects heads, then parses arguments, and finally parses adjunctions.