Stepper protocol

[Eric Dean Tribble <tribble>]

I keep getting tempted, but I'm uncomfortable with pre-incrementing
(pre-Steppers).  I expect to hand (copies of) Steppers from one
routine to another.  Post-Steppers are easier to keep track of in
those situations.  They are also more modular.  If A passes B a
pre-Stepper, B can observe where A left off, or if A used the Stepper
at all.  This is not paranoia.  I can imagine some uses for this that
would feel elegant, but be almost impossible to read or explain
afterwards.

dean