(find-theory <object> [<implementation>] [<error-p>])
Looks for a theory associated with <object>. <object> may be a theory in which case it is simply returned, a symbol which names a theory, or a string which is the printed representation of such a symbol.
If <object> is a symbol or string, a theory named by <object> and associated with implementation <implementation> is sought.
<implementation> should be a symbol (usually a keyword); if unspecified, the current implementation is assumed (see IN-IMPLEMENTATION.)
If IMPLEMENTATION is NIL (not the same as the default), then any theory matching the OBJECT is acceptable (i.e., the theory returned can be in any implementation). <error-p> determines what action will be taken if an appropriate theory cannot be found: if <error-p> is nil FIND-THEORY will return nil; otherwise, an error will be signalled.