In formalisms such as PATR II the string associated with a derivation is the sequence of terminal nodes of the corresponding derivation tree in left-to-right order. For example, the sentence
may be analyzed in some PATR grammar in a way that gives rise to the derivation tree in figure 4.1.
In sign-based approaches such as in UCG and HPSG the string is part of an attribute of each feature structure (sign). The attribute is usually called `phon', `string', `graph' or `orth' (I will use `phon' in the following). Hence, the string associated with a construction is simply the value of the `phon' feature of the sign that is assigned to the construction. In UCG there is a condition, called `adjacency', which says that signs can combine only if they are adjacent. In other words, the value of the `string' feature of a mother node in a parse tree is always the concatenation of the `string' features of the daughter nodes. Hence, the UCG parse tree for the foregoing example presumably would be something like figure 4.2.
The two approaches are formally equivalent, but the second approach has the advantage that it at least becomes easier to think of other `modes' of combination of the value of the `phon' attribute. As an example consider
Suppose that there is linguistic motivation that in this sentence, as in sentence 1, the sequence `easy to please' should be regarded as a (discontinuous) constituent. Such an analysis cannot be defined directly in PATR or UCG. If no adjacency condition applied we could have a parse tree of `easy person to please' as in figure 4.3.
In the next subsections I describe some proposals which allow such a direct implementation of discontinuous constituents.