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## Constraints

The formulas of the logic (constraints) are built from a set V of variables, a set C of constants and a set L of labels (also called attributes or features). Variables are thought of as referring to some specific feature structure. The labels are thought of as pointing to a specific part of a feature structure. A path will be a (possibly empty) sequence of such labels. Such a path can be viewed as the `address' of some piece of information in a specific feature structure. For example the path syn agr person will point to the person part of the agr part of the syn part.

A descriptor is a sequence sp where s is either a variable or a constant, and p is a (possibly empty) path. I write to refer to empty paths. Examples of descriptors are

assuming that X0 is a variable, li are labels and c is a constant. 2.1

Atomic constraints (also called path equations) are equations of the form:

where d1 and d2 are descriptors. For example, the following expressions are atomic constraints, assuming that {X1,X2} V, {syn,agr,number} L and {singular} C:

An -constraint is a set of such atomic constraints, i.e. a set of path equations. The equations in such a constraint are interpreted conjunctively. I write such a constraint as a sequence of equations separated by commas:

,...,

In the following, d is used to refer to descriptors in general, Xi refers to variables, li refers to labels, ci refers to constants, s and t refer to either constants or variables and p and q refer to paths.

Next: Feature graphs Up: The constraint language: Previous: The constraint language:
Noord G.J.M. van
1998-09-30