### Language changes

This section enumerates the changes that have been made to Scheme since the “Revised^4 report” [R4RS] was published.

- The report is now a superset of the IEEE standard for Scheme [IEEEScheme]: implementations that conform to the report will also conform to the standard. This required the following changes:
  - The empty list is now required to count as true.
  - The classification of features as essential or inessential has been removed. There are now three classes of built-in procedures: primitive, library, and optional. The optional procedures are ‘`load`’, ‘`with-input-from-file`’, ‘`with-output-to-file`’, ‘`transcript-on`’, ‘`transcript-off`’, and ‘`interaction-environment`’, and ‘`-`’ and ‘`/`’ with more than two arguments. None of these are in the IEEE standard.
  - Programs are allowed to redefine built-in procedures. Doing so will not change the behavior of other built-in procedures.
- Port has been added to the list of disjoint types.
- The macro appendix has been removed. High-level macros are now part of the main body of the report. The rewrite rules for derived expressions have been replaced with macro definitions. There are no reserved identifiers.
- ‘`Syntax-rules`’ now allows vector patterns.
- Multiple-value returns, ‘`eval`’, and ‘`dynamic-wind`’ have been added.
- The calls that are required to be implemented in a properly tail-recursive fashion are defined explicitly.
- `‘`@`’' can be used within identifiers. `‘`|`’' is reserved for possible future extensions.