Crossings
Pairs of arcs a < b < c < d with a~c and b~d.
Crossings(partition)
Details
- Defined over `SetPartition` as an expression in `_x`, evaluated by compute-engine — the definition IS the implementation.
- Not yet typed over its carrier: `SetPartition` is a restricted growth string in @enumeratio/domains but a list of BLOCKS here, so the head still takes the bare blocks until the two representations are reconciled.
- Read off the standard arc representation: within each block, consecutive elements are linked, and a crossing is two arcs whose spans interleave rather than nest or sit apart.