### 8.4 Compiled and Implementation-Specific Features ¶

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` in `implementation-invicinity` exists, it is loaded. That should produce the file `implcat` in `implementation-invicinity`, 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.