title: Building a particular vector of names in R
body: <p>I have <code>n*m</code> variables of the form <code>x = (i,j,a,b)</code> where <code>i,j=1,...,n</code> and <code>a,b=1,...,m</code>.
  We always have <code>i&lt;=j</code> and <code>a&lt;=b</code>.</p>
  <p>They form the rownames of a (sparse) matrix I want to build in R.
  For example, the first row would be called <code>1,1,1,1</code> and the second one would be <code>1,1,1,2</code>.
  No row will be called <code>2,1,1,1</code> since <code>i&lt;=j</code> and similiarly no row will be called <code>1,2,3,2</code> since <code>a&lt;=b</code>.</p>
  <p>How can I automatically set these rownames in the matrix. The order is not important.</p>