title: What does it mean to "close over" something?
body: <p>I'm trying to understand closures, but literally <strong>every</strong> definition of a closure that I can find uses the same cryptic and vague phrase: "<em>closes over</em>".</p>
  
  <p>What's a <em>closure</em>? "<em>Oh, it's a function that <strong>closes over</strong> another function.</em>"</p>
  
  <p>But nowhere can I find a definition of what "closes over" means. Can someone explain what it means for Thing A to "close over" Thing B?</p>