" Like the return and break keywords, next may be used alone, or it may be followed by an expression or a comma-separated list of expressions. When next is used in a loop, any values following nextare ignored. In a block however the expression or expressions become the "return value" of the yield statement that invoked the block." (The Ruby Programming Language, David Flanagan & Yukihiro Matsumoto, 2008, page 150)

这里说的 next的第一种用法，也就是只用在 loop 中，而不是用在 block 中，什么时候会发生？