TUPLE: entity  < identity-tuple id attribute ;
TUPLE: edge    < entity parent start-pvertex end-pvertex geometry ;
TUPLE: pvertex < entity next-pvertex parent vertex ;

: edge-pvertices ( edge - pvertices )
    {  [ start-pvertex>> ] [ end-pvertex>> ] bi [ 2dup eq? not ] [ over next-pvertex>> swap ] while }