title: About the xsl processing
body: <p>I am using xsl to transform a xml to html. A table is created in the html but some problem of the unduplicated items appear.
  If I use a unique key to form the table(xsl for-each of every row), for example, context, but this attribute is not in the table.</p>
  
  <p>the final table is like this</p>
  
  <pre><code>context(invisible)  attr1  attr2 attr3
  1                     A      b    x
  2                     A      b    x
  3                     A      c    x
  4                     B      a    x
  </code></pre>
  
  <p>Now at the same time, I want to use attr1&amp;attr2 both as another primary key, which means that item 1 and item 2 is just the same(one will be eliminated automatically during the xsl processing), How xsl achieve this goal?</p>