title: What is a "receiver" in Kotlin?
body: <p>How is it related to extension functions? Why is <code>with</code> <a href="https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/with.html" rel="noreferrer">a function</a>, not a keyword?</p>
  
  <p>There appears to be no explicit documentation for this topic, only the assumption of knowledge in reference to <a href="https://kotlinlang.org/docs/reference/extensions.html" rel="noreferrer">extensions</a>.</p>