fair conjunction notes

gistfile1.txt

Streams were equipped with plus and bind, I removed bind and added join and narrow.
I also added yield which returns the yielded substitution map for the stream (if any) 
and step which advances teh computation (was: calling the thunk).

join, like plus, interleaves execution of its two streams. As soon as one yields, the
yielded value is used to narrow the search space of the other stream.

For any stream A, we have:
  A = (plus (yield A) (step A))

The interleaving formula behind join is:
  (join A B) = (plus (narrow B (yield A)) (join B (step A))) ; interleaving on the join term

narrow is distributive over plus and join
  (narrow (join A B)) = (join (narrow A) (narrow B))
  (narrow (plus A B)) = (plus (narrow A) (narrow B))

Implementation detail
narrowed streams remember the substitutions map (the "min-yield") which was used to
narrow them so whole streams can be pruned if one tries to narrow a stream with a
substitution incompatible with its min-yield.