#
Notes

### Language changes

This section enumerates the changes that have been made to Scheme since the ``Revised4 report'' [6] was published.

- The report is now a superset of the IEEE standard for Scheme [13]: 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.