Gizmo: accumulate results in map() about cayley

[barakmich]

This is a bug in the Emit function I think, combined with not accumulating the return values on ForEach (where not undefined), which is straightforward enough.

[dennwc]

There is no error on HEAD for first example.
Will implement accumulation a bit later.

[iddan]

Correction: .forEach() shouldn't accumulate results, .map() should.

[iddan]

Closing this as #878 will not include steps that accept a function.