Negation is introduced through negation of a relation. For example, the screwdrivers on the table that are not philips screwdrivers, would be expressed as:

Desc: (SCREWDRIVER S (NOT-TYPE PHILIPS) (ON TABLE T))
Alist: (T TABLE-1 )

     In this case, the variable S is unbound and the specific description roughly corresponds to the English statement, "Any screwdriver that is not a philips and is on table-l."

     The requirement that an instance be included in more than one class is expressed as a template conjunct which is simply a list of templates. A particular object that is both a picture frame and an antique could be described as:

Desc: ((PICTURE-FRAME ANTIQUE) P)
Alist: (P PICTURE-FRAME-1 )

     Finally, the language allows a shorthand construction called a rel-path. One might wish to describe the screwdrivers that are on anything that is in turn on the table. This would be expressed as:

Desc: (SCREWDRIVER S ((ON ON) TABLE T))
Alist: (T TABLE-1 )

     The Descform differs from a logical expression in the sense that the order of the expression has computational significance. Intuitively the idea is that the typed variable or instance that is mentioned at the beginning of the description is the focus of that description. This entity is referred to as the root or source of the relation. The list of rspecs describe relationships between this root and other instances or Descs. Since the target itself can be a Desc, an unlimited amount of embedding is allowed. An example of embedding is given by the description:

Desc: (SCREWDRIVER S (ON (TABLE T) (COLOR RED))))

which describes any screwdriver on a red table.

     The fact that these are ordered does not affect the standard semantic interpretation of a specific description. A semantic interpretation of a specific description is obtained by matching the forms in the description against the assertions in WM. If bindings are provided for each variable in the description and if the assertion and appropriate truth value for each of the subexpressions is found in WM, then the match succeeds and the specific description is true of that context. If some variables in the expression are unbound, and if the match succeeds, then the bindings that made the description true are also returned. In this way a query can also returns additional information that further specifies the description.

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