R5Rs Scheme Revised(5) Report on the Algorithmic Language Scheme -- Notes

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### 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.

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