From Gambit wiki
Particularities of Gambit-C Scheme
Some extensions to the Scheme programming language are introduced below. This (non exhaustive) list is intended for the users who want to know more about the features of Gambit without parsing the whole documentation.
- To allow user-defined types and structures, with many enhancements to enable inheritance, read-only fields and others options, see the define-type page.
- Modular hash-tables are provided, where you can select your hash functions, your equality predicates, use weak pointers, etc.
- What about some extra parameters such as keys, optional parameters, references to whole parts of code, etc?