predefined unary operator to test emptiness of a channel.
a channel name, is equivalent to
len(q) != 0 .
The Promela grammar prohibits this from being written as
instead of its equivalents can preserve the validity
of an addition partial order reduction during verifications,
that can be defined with
xr(2), xs(2), empty(5), full(5), len(5), nfull(5).