Customizing the metamodel : Keywords for USRPROPS : WHERE
  
WHERE
Displays only those definitions in the Choices dialog that contain a fixed value in a named property of the definition.
Example
Rename Definition "User 1" To "Aircraft Type" Rename Definition "User 2" To "Filtered Aircraft"
List "Engine" { Value "Propeller" Value "Jet" Value "Glider" }
Definition "Aircraft Type" { Property "Engine Type" { EDIT Text List "Engine" Length 48 }
}
Definition "Filtered Aircraft" { Property "Selected Aircraft Type" { edit listof "Aircraft Type" WHERE "Engine Type" = "Jet"} }
If the above USRPROPS.TXT were applied to an encyclopedia, and the following Aircraft Type definitions were created in the encyclopedia:
Mustang (engine = Propeller) Spitfire (engine = Propeller) F-16 Fighting Falcon (engine = Jet) F-86 Sabre (engine = Jet)
then when you create a definition of type Filtered Aircraft (named Current Jet Fighters, for example), clicking Choices reveals only two choices: F-16 Fighting Falcon and F-86 Sabre; all definitions with Engine Type set to Propeller do not appear in the Choices list.