title: OCaml: Can't run utop after installing it
body: <p>I'm trying to learn OCaml through the Real World OCaml book. They have a guide by which I am supposed to install the Core package and utop. However, while I seem to be successfully installing both of these using Opam, neither of them works when I try to use them. </p>
  
  <p>I know that they're installed, because when I try to install them again, I get this message: </p>
  
  <pre><code>$ opam install utop core
  [NOTE] Package utop is already installed (current version is 1.10).
  [NOTE] Package core is already installed (current version is 109.55.02).
  </code></pre>
  
  <p>However, when I try to enter "utop" to start utop, it doesn't work. </p>
  
  <pre><code>$ utop
  bash: utop: command not found
  </code></pre>
  
  <p>Same when I try to open core:</p>
  
  <pre><code>$ open Core.Std
  Couldn't get a file descriptor referring to the console
  </code></pre>
  
  <p>What's going on here? I'm new to programming so there might be something about installation that I'm just not understanding. </p>