The `current-environment-variables' parameter determines the current
mutable "environment variable set". If that set is the initial one for
a Racket process, then using the set corresponds to working with OS
environment variables. Otherwise, it's really just a hash table that
is packaged up as OS environment variables if a subprocess is created.
The new environment-variable interface works in terms of bytes, instead
of assuming that environment variable names and values fit in a string
encoding.
The string-based `getenv' and `putenv' are still available as
convenience wrappers. The checking on environment-variable names
for those wrappers is a little tighter, preventing any attempt to use a
name that contains "=".
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