function soundProp(input, output) {
  return isSubset(_.flatten(input), _.flatten(output));
}
## Verifying model

  We will use properties described in [the blog post by Jessica Kerr](http://blog.jessitron.com/2013/04/property-based-testing-what-is-it.html).
  Actually, the idea of this example is originated from that post.

  ### 1. Every element in the input is also in the output