Often an implementation can implement an SLIB feature more efficiently than the R4RS-compliant source code in SLIB. Alternatively, implementations with compilers can compile SLIB source code into binary files which run faster than loading source code.
Additionally, the SLIB catalog can be augmented with extra-SLIB features which can be loaded by the implementation. The catalog format is described in See Library Catalogs.
These implementation-specific modifications are made when a new
catalog is created (see Catalog Creation). If mkimpcat.scm
implementation-invicinity exists, it is loaded. That should
produce the file implcat in
whose associations will override those of SLIB. implcat is
copied into slibcat in
implementation-vicinity as part
of the catalog creation process; modifications to implcat after
that will have no effect.