Essential and non-essential features

It is required that every implementation of Scheme support features that are marked as being essential. Features not explicitly marked as essential are not essential. Implementations are free to omit non-essential features of Scheme or to add extensions, provided the extensions are not in conflict with the language reported here. In particular, implementations must support portable code by providing a syntactic mode that preempts no lexical conventions of this report and reserves no identifiers other than those listed as syntactic keywords in section .