Novice Question: Implement the Step functionality in VBA

[freeflow]

I'm converting Advent of Code solutions from VBA (twinBasic) to Nim

I have a bit of a headscratcher when it comes to implementing the Step 
functionality in 'for x to y step z' because in VBA step can be a positive or a 
negative value wheres nim uses countup or countdown iterators with the step 
value always being a positive number
    
    
    Dim myXStep As Long = If(s.X1 <= s.X2, 1, -1)
     For myXCoord = s.X1 To s.X2 Step myXStep
    
    
    Run

I tried being clever with this code in nim
    
    
    var myXStep:iterator [T](a, b:T): T   = if s.X1 <= s.X2: [T]countup(a,b:T) 
else: [T]countdown(a,b:T)
    
    
    Run

so that I could then write
    
    
    for myX in myXstep(s.X1,s.X2)
    
    
    Run

but it doesn't work, even when I convert the 1 line version to a multiline 
version

I have elsewhere successfully used something similar to the above to provide 
procs as a parameter but I'm obviously not quite there yet with the above 
attempts.

Advice would be welcome.