code: func Concat(xs interface{}) interface{} {
  	chk := ty.Check(
  		new(func([][]ty.A) []ty.A),
  		xs)
  	vxs, tflat := chk.Args[0], chk.Returns[0]
  
  	xsLen := vxs.Len()
  	vflat := reflect.MakeSlice(tflat, 0, xsLen*3)
  	for i := 0; i < xsLen; i++ {
  		vflat = reflect.AppendSlice(vflat, vxs.Index(i))
  	}
  	return vflat.Interface()
  }
docs: // Concat has a parametric type:
  //
  //	func Concat(xs [][]A) []A
  //
  // Concat returns a new flattened list by appending all elements of `xs`.