Wut

type
  CoinRef = ref RootObj
  PenRef  = ref RootObj

method update(o: RootObj) = discard
method update(p: PenRef)  = echo "Im a pen"
method update(c: CoinRef) = echo "Im a coin"

let s: seq[ref RootObj]   = @[CoinRef(), PenRef(), CoinRef(), PenRef()]

for i in s: update i

# Result:
# >Im a pen
# >Im a pen
# >Im a pen
# >Im a pen