nilを得る十一種類の方法
x1 = (begin end)
x2 = (BEGIN{10})
x3 = (END{20})
x4 = (def foo; end; foo())
x5 = $12345
x6 = eval('')
x7 = Class <=> self
x8 = [][1]
x9 = Object.superclass
x10 = initialize()
x11 = ObjectSpace._id2ref(4)
p(x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,x11)