sphere surf 1
convert surf surf
mkface f surf
getanasurf asurf f sph  1.e-7
if {[isdraw asurf]} {
  set log [dump asurf]
  if { [regexp {SphericalSurface} $log ] != 1 } {
     puts "Error: surface is not a spherical surface"
  }
} else {
  puts "Error: required surface is not got"
}