Returns a list of environment variable names that are suitable for passing to
fn:environment-variable, as a (possibly empty) sequence of strings.
The function returns a sequence of strings, being the names of the environment variables in the dynamic context in some implementation-dependent order.
The function is deterministic: that is, the set of available environment variables does not vary during evaluation.
The function returns a list of strings, containing no duplicates.
It is intended that the strings in this list should be suitable for passing to
See also the note on security under the definition of the
fn:environment-variable function. If access to environment variables has
fn:available-environment-variables always returns the empty