title: Difference between "free variable" and "free occurrence of a variable" in context of lambda calculus
body: <p>Is there any difference between free variable and free occurrence of a variable in context of lambda calculus? If yes, then please explain with an example or two. 
  Actually I was going through the conversion rules for lambda expression where I came across the following line:</p>
  
  <blockquote>
    <p>In stating the conversion rules the notation <code>E[E'/V]</code> is used to mean the result of substituting <code>E'</code> for each free occurrence of <code>V</code> in <code>E</code></p>
  </blockquote>