The following GFA BASIC for Atari ST code was translated in p5.js :
N=0
X=Xrsl%
Y=Yrsl%
'
Repeat
D=D+1
Defline 1,D
N=N+D+1
X=X-D-10
Y=Y-D-10
Draw N,N To N,Y To X,Y To X,N To N,N
Until N>=Y
'
Pause 100