Sat Sep 3 10:40:44 EDT 2016

Sequences as left folds

I've converted the default abstraction in the project to sequences
represented as left folds.

This works a lot better than sequences as sink-parameterized
generators because it is easier to compose.

The reason is _stateful_ processing of sequences.  Composition works
by taking an input fold, and have it fold over some compound state,
then filter that out again in the final result.

To convert to sink-parameterized generators, simply include the sink
in the body of a fold operation.