: edge-pvertices ( edge -- pvertices ) [ end-pvertex>> ] [ start-pvertex>> ] bi 2dup eq? [ drop 1array ] [ ! ( end start ) dup -rot ! prepare for loop ! ( start end start ) [ 2dup eq? not ] [ [ next-pvertex>> ] keep ] produce ! generate partial pvertices sequence (start is missing) ! ( start end end seq ) 4 -nrot 2drop prefix ! prefix start on seq and remove the two end values ] ! ( seq ) if ;