A keyword indicating the database type to initialize.
Either t if the initialization succeeds or nil if it fails.
This generic function implements the main part of the database type initialization performed by initialize-database-type. After initialize-database-type has checked that the given database type has not been initialized before, as indicated by *initialized-database-types*, it will call this function with the database type as it's sole parameter. Database back-ends are required to define a method on this generic function which is specialized via an eql-specializer to the keyword representing their database type.
Database back-ends shall indicate successful initialization by returning t from their method, and nil otherwise. Methods for this generic function are allowed to signal errors of type clsql-error or subtypes thereof. They may also signal other types of conditions, if appropriate, but have to document this.