NewFeature < Main

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---+Grammar Engineering Frequently Asked Questions

---++I'm trying to add a new feature, and the LKB doesn't like it.  What should I do?

There are two rules about where you can declare new features:


   * Each feature can only be declared for one type. This means you can't reuse feature names.
   * Features can only be declared for a type on the definition of a type.  This means that new features are always features of the outermost feature structure in the type definition in which they are declared.


---+++"No possible type ..."

If you're getting the error message "No possible type for features (...) at path (...)" you have probably tried to introduce a feature other than in the outermost feature structure.  For example, if you wanted to add a feature CASE appropriate only for nouns, the correct way to do it is on the type =noun= (subtype of =head=):

<verbatim>
noun := head &
   [ CASE case ].
</verbatim>

If instead, you tried to do it on =noun-lex=:

<verbatim>
noun-lex := basic-noun-lex & spr-only-lex-item &
   [ SYNSEM.LOCAL.CAT.HEAD noun &
                           [ CASE case ] ].
</verbatim>

you would get this error message.  (If you also have the correct declaration on =noun=, the constraint shown here on =noun-lex= won't cause an error, but it will be redundant.)

---+++"Feature ... introduced at multiple types (...)"

As stated above, features can only be declared for one type.  If two types need the same feature, they need to have a common supertype from which to inherit it.  You might hit this error if you happen to try to reuse a feature name already in use in the matrix, or if you try to declare a feature for two types at the same time (e.g., FORM on =verb= and =prep=) instead of putting it on a supertype.

NB: You can of course mention features on multiple types, and often need to in order to constrain their values appropriately.  They just need to be introduced on a supertype to all types that they are appropriate for. This doesn't mean that e.g., =noun-lex= needs to inherit from =noun= in order to constrain a value of CASE.  In the example above, CASE remains a feature of =noun=, which happens to be the value at the end of a path inside =noun-lex=.

---+++Adding a feature to an existing type

If you want to add a feature to a type that's already declared in
the matrix, you need to write a [[TypeAddendum][type addendum statement]].

---

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-- EmilyBender - 02 Nov 2004
Topic revision: r4 - 2005-05-19 - EmilyBender