title: Split multiple fields per line to separate lines using sed, retaining line prefix
body: <p>Last Friday I got a problem, to transform a text to another format. On that machine, only gnu sed is available, no awk (strange, I know). And I know nothing about perl. so I am looking for a sed only solution.</p>
  
  <p>the file content is:</p>
  
  <pre><code>a  yao.com sina.com
  b  kongu.com
  c  polm.com unee.net 21cn.com iop.com foo.com bar.com baz.net happy2all.com
  d  kinge.net
  </code></pre>
  
  <p>the required output, (should be a new file) is:</p>
  
  <pre><code>a  yao.com 
  a  sina.com
  b  kongu.com
  c  polm.com 
  c  unee.net 
  c  21cn.com 
  c  iop.com
  c  foo.com
  c  bar.com
  c  baz.net
  c  happy2all.com
  d  kinge.net
  </code></pre>
  
  <p>I tried a lot, also searched famous sed oneliner, but I cannot make it... can someone help me?</p>