Scheme implementations compliant with the report may operate in a variety of modes. In particular, in addition to one or more modes conformant with the requirements of the report, an implementation may offer non-conformant modes. These modes are by nature implementation-specific, and may differ in the language and available libraries. In particular, non-conformant language extensions may be available, including unsafe libraries or otherwise unsafe features, and the semantics of the language may differ from the semantics described in the report. Moreover, the default mode offered by a Scheme implementation may be non-conformant, and such a Scheme implementation may require special settings or declarations to enter the report-conformant mode. Implementors should clearly document the nature of the default mode and how to enter a report-conformant mode.