polyline p 0 0 0 4 0 0 4 0 2 2 0 4 0 0 4 0 0 0
mkplane f p
prism s f 0 5 0
explode s f
donly s
offsetparameter 1e-7 c i
offsetload s 1 
offsetonface s_2 5
offsetperform result

checkprops result -v 269.397

checknbshapes result -shell 1
