Sometimes we need to pick the special structure with large space. But the EXTERNAL command always include pairs of edges between which other polygons exist. And EXTERNAL has no option to filter these edges.
Herewith below is my suggestion for pick large space edge, please help me double check and i hope you can share a better way.
a = external [layer] >= a <=b opposite
b = external [layer] < a opposite
c = not coincident edge a b