I am reading "Revised^5 Report on the Algorithmic Language Scheme," "1.3.1. Primitive, library, and optional features."
In particular, implementations must support portable code by providing a syntactic mode that preempts no lexical conventions of this report.
I don't understand this passage. Maybe it's because English is not my native language. My dictionary does not offer a meaning of the verb "preempt" that makes the passage meaningful. A little clarification would be appreciated.