alts! is a function that accepts a vector of channels to take
from and/or channels with values to be put on them (in the form of
doubleton vectors: [c v]). The vector may be dynamically constructed;
the code calling alts! may not know how many channels it'll be choosing
among (and indeed that number need not be constant across invocations).
alt! is a convenience macro which basically acts as a cross between
cond and alts!. Here the number of "ports" (channels or channel+value

