title: what is 'calldepth' parameter for in logger.Output() method in log package?
body: <p>In document <a href="https://golang.org/pkg/log/#Logger.Output" rel="nofollow noreferrer">https://golang.org/pkg/log/#Logger.Output</a>,</p>
  <p>there is <code>calldepth</code> argument in <code>logger.Output()</code> method.</p>
  <p>I played with example code in doucment and it seems like when I pass higher value, logger print out lower location logging event occured.</p>
  <p>When I pass 1 as calldepth param, I understand <code>logger.Output()</code> script is written in exact line 15.</p>
  <pre><code>INFO: prog.go:15: Hello world
  </code></pre>
  <p>But when I pass 2, I got below.</p>
  <pre><code>INFO: prog.go:19: Hello world
  </code></pre>
  <p>Why line number is different even though file name is both <code>prog.go</code>?</p>