Create or find a restriction.
restriction name [site [:if-exists y-action][:if-does-not-exist n-action]]
Get or create a restriction for the name.
The :if-does-not-exist keyword control what happens when a matching restriction doesn't exist. By default, the function returns NIL. If the keyword value is :create, the function creates and returns a new restriction.
The :if-exists keyword controls what happens when the restriction already exists. By default, the function returns the restriction. If the keyword value is :error, the function will signal an error.
The returned value is a restriction object or NIL.
Interaction Developer Reference