Test Driven Definition: Behaviour assertions in decorators example.

decorator_assertions.rb

def must_contain_plus meth                                                                                      
  raise unless RubyVM::InstructionSequence.disasm(method(meth)).include? 'opt_plus'
  meth
end

must_contain_plus(
def add(x, y)
  x + y
end)
#=> :add

must_contain_plus(
def mult(x, y)
  x * y
end)
#=> RuntimeError: 
#   from (pry):21:in `must_contain_plus'