NOTE FROM RALPH
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This model for 2D flows is adapted from Vector Field.nlogo by Uri Wilensky
by changing the definition of the field only.

This is identical to the cusp family, Experiment #2, except we now have
have a switch "direction" to reverse the flow.
Attractors of the direct flow become repellors of the reversed flow,
and visa versa.

Try drawing the attractor and basin portrait with a = -1000, and several
values of b between -15,000 and +15,000. In each case, how many attractors
and basins to you find? And, try to locate the repellors exactly.

See the instructions for Experiment #2 for more explanation.