You made a small mistake in your first `+=` proc.
What you _want_ to do in a proc such as that is _mutate_ the existing input,
not produce an output.
You want to change the existing `VGALine` in `a` (and by extension: `line`)
instead of making an entirely new one, right? You did the opposite and created
a new one (so line += 1 creates a new VGALine value).
And since nim doesn't allow you to use a proc that returns something without
_explicitly saying_ that you don't want it to return something, this is the
compiler basically refusing to compile something that it is betting on is an
accident.
Try to replace you `+=` proc for VGALine with something like this:
proc `+=` (a: var VGALine, b: int) =
a = ((a.int+b) mod 80).VGALine
Run
