y0 :> y!
y0 y1 < [ 1 ystep! ] [ -1 ystep! ] if
x0 x1 1 <range> [
- y steep [ swap ] when 2array
- current-error deltaerr + current-error!
+ y steep [ swap ] when 2array
+ current-error deltaerr + current-error!
current-error 0.5 >= [
ystep y + y!
current-error 1 - current-error!