Am iterating over this sequence correctly?

[ninovanhooff]

I'm trying to modify a sequence in-place by modifying the values without 
copying the data.

This worrks, but I wonder wether I'm doing it in an idiomatic 'Nim' way, rather 
than trying to use `ptr` too much.

I feel the answer should be here, where it is suggested to use `ref`. Not sure 
how to get that working tho: <https://forum.nim-lang.org/t/6457>

tl;dr In the code below, I expect that I should use `ref seq[Vertex]` while 
`ptr` is meant for interfacing with C and such. Is that correct, and if so, how 
to modify the snipppet of code?

types
    
    
    type
        Int32x2 = array[2, int32]
        Vertex* = Int32x2
        Polygon* = object of RootObj
            vertices*: seq[Vertex]
            fill*: LCDPattern
    
    
    Run

code
    
    
    proc loadPolygon(level: Level, obj: LevelObjectEntity): bool =
      let objOffset: Vertex = [obj.x, obj.y]
      var polygon: Polygon = obj.getPolygon()
      var vertices: ptr seq[Vertex] = addr polygon.vertices
      let lastIndex = vertices[].high
      
      if lastIndex < 2:
        return false # polygons require at least 3 vertices
      
      # Offset the polygon by the object's position (localToWorld)
      for i in 0..lastIndex:
        vertices[i] = vertices[i] + objOffset
      [..]
    
    
    Run