Scheme, as defined in R4RS, does not have a module system at all.
Aubrey Jaffer, mostly to support his portable Scheme library SLIB, implemented a provide/require mechanism for many Scheme implementations. Library files in SLIB provide a feature, and when user programs require that feature, the library file is loaded in.
For example, the file `random.scm' in the SLIB package contains the line
so to use its procedures, a user would type
and they would magically become available, but still have the same names! So this method is nice, but not as good as a full-featured module system.
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