For next release:

+ Move all of the legacy-adapter stuff to a new LegacyAdapter directory.
  Use David's boingeeFieldTrip adapter as a basis.  Separate out all of
  the GLUT stuff.

+ Much-simplified interfaces.  FRP.Reactive has just behaviors and events.
  FRP.Reactive.LegacyAdapter for building legacy adapters.

+ Something for accessing representations but not exposed in FRP.Reactive.

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Beyond:

+ Revisit choice of time as double.  Consider nanoseconds or microseconds
  instead, for regularity of time-stepping.

+ Semantics-based fusion rules.  Inquire about fusion vs sharing.