Module SSWP := WPropertiesOn SetOfSetsOfPositiveOrderedType SS.
Module SSWEqP := WEqPropertiesOn SetOfSetsOfPositiveOrderedType SS.
Module SPWEqP := WEqPropertiesOn SetOfPairsOfPositiveOrderedType SP.
Definition pick_line (s : SS.elt) (sp : SP.t) :=
SP.exists_ (fun p => (S.mem (fstpp p) s) && (S.mem (sndpp p) s)) sp.

Lemma proper_0 :
Proper (S.Equal ==> eq ==> eq) pick_line.