Methods that simply behave differently depending on whether or not an argument is given. For example, passing nil (or any other object) to inject is different f

Methods that simply behave differently depending on whether or not an argument is given. For example, passing nil (or any other object) to inject is different from passing nothing. [1, 2, 3].inject(:+) [1, 2, 3].inject(0, :+) [1, 2, 3].inject(nil, :+) # doesn't work Rubinius uses a magic ``undefined`` value for this: def inject(initial=undefined, sym=undefined, &block)