Practical Grammar 8

The Difference between Defining Equations and Constraining Equations

There are several types of equations that can be used in annotations. So far, we have encountered the following two:

(1) ↑=↓1;
(2) (↑ OBJ) =↓2;

These equations are both defining equations.

Defining equations add their information to an f-structure.

There is a second kind of equation, which we have not seen yet, but which you will need for the following exercise. These are called constraininig equations.

Constraininig equations test whether their information is contained in an f-structure. They do NOT add the information themselves.

Illustration:

Case 1:

Imagine you have the following defining equation:

(↑ TENSE) = pres;

• it turns the f-structure [] into the f-structure [TENSE pres], i.e. it adds its information to the f-structure.
• it turns the f-structure [TENSE pres] into the f-structure [TENSE pres], i.e. it adds its information to the f-structure. If the information was already there, the f-structure remains the same.

Case 2:

Now, imagine you have the following constraining equation:

(↑ TENSE) =_c pres;

• it marks the f-structure [] as ill-formed, since it does not contain the information TENSE pres, i.e. the constraining equations is a test on an f-structure.
• it marks the f-structure [] as well-formed, but does not change it.

When to use a constraining equation:

Constraining equations are useful when one item needs to make sure that an f-structure contains a particular piece of information that must be contributed by some other item.