test a bug-prone formulation of reduce
test a bug-prone formulation of reduce

this formulation is susceptible to an a-b-a problem that was solved in 38c56c6.
(the implementation of reduce was changed in c46a9b9 for a different reason;
the newer formulation is less susceptible to the a-b-a problem.)

glessard/async-deferred,glessard/deferred,glessard/deferred,glessard/swiftian-dispatch