>> force (Thunk.new {1} + 1)
=> 2
>> force (Thunk.new {1} + Thunk.new {1})
TypeError: coerce must return [x, y]
	from ./foobar.rb:11:in `+'
	from ./foobar.rb:11:in `__send__'
	from ./foobar.rb:11:in `method_missing'
	from ./foobar.rb:15:in `call'
	from ./foobar.rb:15:in `__force__'
	from ./foobar.rb:57:in `force'
	from (irb):15