How to use concept in Nim

concepts.nim

type
  CanDance = concept x
    dance(x) # `x` is anything that has a `dance` procedure

proc doBallet(dancer: CanDance) =
  # `dancer` can be anything that `CanDance`
  dance(dancer)

# ---

type
  Person = object
  Robot = object

proc dance(p: Person) =
  echo "People can dance, but not Robots!"

# ---

let p = Person()
let r = Robot()

doBallet(p) # works. prints: "People can dance, but not Robots!"
doBallet(r) # ERROR: (expected a type which CanDance)