The mechanism above is sufficient to create an acceptable module
system. There are however cases in which we would like to be able to
overrule this schema and explicitely call a predicate in some module
or assert explicitly in some module. The first is useful to invoke
goals in some module from the user's toplevel or to implement a
object-oriented system (see above). The latter is useful to create
and modify dynamic modules (see section 
).
For this purpose, the reserved term :/2 has been introduced.
All built-in predicates that transform a term into a predicate
reference will check whether this term is of the form
`<Module>:<Term>'. If so, the predicate is searched
for in Module instead of the goal's context module. The
:/2 operator may be nested, in which case the inner-most module
is used.
The special calling construct <Module>:<Goal> pretends Goal is called from Module instead of the context module. Examples:
?- assert(world:done). % asserts done/0 into module world ?- world:assert(done). % the same ?- world:done. % calls done/0 in module world