polyline w 0 0 0 1 0 0 1 1 0 0 1 0 0 0 0
getanasurf res w pln  1.e-3
if {[isdraw res]} {
  set log [dump res]
  if { [regexp {Plane} $log ] != 1 } {
     puts "Error: surface is not a plane"
  }
} else {
  puts "Error: required surface is not got"
}