title: Usage example for Format method with leading zero padding for Big Ints in Golang
body: <p>I would like to format a big int as a string with leading zeros.  I'm looking for an example similar to <a href="https://stackoverflow.com/questions/25637440/how-to-pad-a-number-with-zeros-when-printing">this</a>, but with Big:</p>
  
  <p>I'm looking at source <a href="https://golang.org/pkg/math/big/#Int.Format" rel="nofollow noreferrer">here</a>.</p>
  
  <p>But when I call:</p>
  
  <pre><code>m := big.NewInt(99999999999999)
  fmt.Println(m.Format("%010000000000000000000","d"))
  </code></pre>
  
  <p>I see:</p>
  
  <pre><code>prog.go:10:22: m.Format("%010000000000000000000", "d") used as value
  prog.go:10:23: cannot use "%010000000000000000000" (type string) as type fmt.State in argument to m.Format:
      string does not implement fmt.State (missing Flag method)
  prog.go:10:48: cannot use "d" (type string) as type rune in argument to m.Format
  </code></pre>
  
  <p>(I understand normally I can use m.String(), but zero padding seems to complicate this, so I'm looking specifically for some help on the Format method.)</p>
  
  <p>Here's <a href="https://play.golang.org/p/7Uz2trkM8Fo" rel="nofollow noreferrer">my playground link</a>.</p>