title: ruby: What's the meaning of keyword "in"
body: <p>When I find the keyword "in" in ruby first time. I think maybe I can do that:
    1 in (0..10)
  But it look like I cannot use it like that way.</p>
  
  <p>Then I search it in ruby-lang.org, and google it. There is no answer!</p>
  
  <p>What's the meaning of keyword "in" in ruby?</p>