# Pastebin Hh663xI3
# copied from https://egonschiele.github.io/contracts.ruby/

$ ruby -v; cat math.rb ; ruby math.rb
ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin17]
require 'contracts'

class Math
  include Contracts::Core

  Contract Contracts::Num, Contracts::Num => Contracts::Num
  def self.add(a, b)
    a + b
  end
end

puts Math.add(1, "foo")
Traceback (most recent call last):
math.rb:3:in `<main>': Math is not a class (TypeError)