code: func In(needle, haystack interface{}) bool {
  	chk := ty.Check(
  		new(func(ty.A, []ty.A) bool),
  		needle, haystack)
  	vhaystack := chk.Args[1]
  
  	length := vhaystack.Len()
  	for i := 0; i < length; i++ {
  		if vhaystack.Index(i).Interface() == needle {
  			return true
  		}
  	}
  	return false
  }
docs: // In has a parametric type:
  //
  //	func In(needle A, haystack []A) bool
  //
  // In returns `true` if and only if `v` can be found in `xs`. The equality test
  // used is Go's standard `==` equality and NOT deep equality.
  //
  // Note that this requires that `A` be a type that can be meaningfully compared.