The imported predicates act as weak symbols in the module into which they are imported. This implies that a local definition of a predicate overrides (clobbers) the imported definition. If the flag warn_override_implicit_import is true (default), a warning is printed. Below is an example of a module that uses library(lists), but redefines flatten/2, giving it a totally different meaning:

:- module(shapes, []).
:- use_module(library(lists)).

flatten(cube, square).
flatten(ball, circle).

Loading the above file prints the following message:

Warning: /home/janw/Bugs/Import/t.pl:5:
        Local definition of shapes:flatten/2
        overrides weak import from lists

This warning can be avoided by (1) using use_module/2 to only import the predicates from the lists library that are actually used in the `shapes' module, (2) using the except([flatten/2]) option of use_module/2, (3) use :- abolish(flatten/2). before the local definition or (4) setting warn_override_implicit_import to false. Globally disabling this warning is only recommended if overriding imported predicates is common as a result of design choices or the program is ported from a system that silently overrides imported predicates.

Note that it is always an error to import two modules with use_module/1 that export the same predicate. Such conflicts must be resolved with use_module/2 as described above.