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# inverse(+Succ, +Pred)

Constrains elements of Succ to be the successors and Pred to be the predecessors of nodes in a digraph
*Succ*
- A collection of N different variables or integers
*Pred*
- A collection of N different variables or integers

## Description

Succ and Pred are list of N elements, representing a digraph of N nodes,
where the i'th element of Succ and Pred represents the successor and
predecessor of the node i respectively. The constraint enforces each
node in the digraph to have one successor and one predessor node, and
that if node y is the successor of node x, then node x is the
predecessor of node y.

This is currently a prototype -- the constraint has not been tested
very extensive and little effort has been spent to optimise performance.
We welcome any feedback on using this constraint.

This constraint is known as inverse in the global constraint catalog,
but with implicit node index based on the position in the list.