<mattmitchell> (doc reduce)
<sexpbot> ⟹ "([f coll] [f val coll]); f should be a function of 2
arguments. If val is not supplied, returns the result of applying f to
the first 2 items in coll, then applying f to that result and the 3rd
item, etc. If coll contains no items, f must accept no arguments as
well, and reduce returns the result of calling f with no arguments. If
coll has only 1 item, it is returned and f is not called. If val is
supplied, returns the result of applying f to val and the first item
in coll, then applying f to that result and the 2nd item, etc. If coll
contains no items, returns val and f is not called."