title: Getting position information from JSON token in source
body: <p>I'm using <code>yojson</code> to parse a JSON file in OCaml, and while I'm validating the input file, I'd like to emit error messages containing source code information for the user.</p>
  
  <p>For instance, if I found that a given value at line 5492 is not in the allowed range, I'd like to tell the user: "Invalid value in line source.json:5492".</p>
  
  <p>How can I get this information?</p>
  
  <p>(If yojson cannot give this kind of information, another OCaml JSON library that does have this information might be useful.)</p>