A possible answer (three procedures needed):
to flag :w
fd 1.5 * :w
square :w
END
to flags :p
if :p < 20 [stop]
flag :p
pu bk :p rt 90 fd 2 * :p lt 90 pd
flags :p - 20
end
to square :size
repeat 4[fd :size rt 90]
end
