Fun with swap in Nim

# This is chopped up to only be the relevant code of course.

iterator mitems*[T](a: var seq[T]): var T {.inline.} =
  var i = 0
  let L = len(a)
  while i < L:
    yield a[i]
    inc(i)
    assert(len(a) == L, "seq modified while iterating over it")

type
  MemoryCell* = object
    id*: int
    data*: CellData

  Memory* = seq[ref MemoryCell]

  Link* = object
    source*: MemoryCell
    dest*: MemoryCell

  Links* = seq[Link]

proc reverse*(links: var Links) =
  # reverses data flow in links
  for link in links.mitems:
    swap(link.source, link.dest) # What happens here? Is this the right way to do what I want? Is this needless copying?
    # If I had refs, would the instance count keep rising with the swap or neatly swap the pointers which is what I want ideally.
    # Am I better off using effects to manage this?