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|Explicit manipulation of the calling context|
Quintus' derived module systems have no means to separate the lookup
module (for finding predicates) from the calling context (for qualifying
meta arguments). Some other Prolog implementations (e.g., ECLiPSe and
IF/Prolog) distinguish these operations, using
setting the calling context of a goal. This is provided by SWI-Prolog,
currently mainly to support compatibility layers.
For example, the code
asserta(done)@world is the same as
asserta(world:done). Unlike in
is resolved in the current module rather than the module
world. This makes no difference for system predicates, but
usually does make a difference for user predicates.
Not that SWI-Prolog does not define
@ as an operator.
Some systems define this construct using
op(900, xfx, @).