// enter program

Paper 100
Pen 0

Repeat A 0 13
{
   Line 0 (100-2*A) 100 (100-2*A)
}

Set B 1
Repeat A 0 20
{

   Set B (B+1)
   Line 0 (72-2*A) (50-B) (72-2*A)
   Line (50+B) (72-2*A) 100 (72-2*A)
}

Repeat A 0 15
{
   Line 0 (30-2*A) 100 (30-2*A)
}