title: S4 class in R for matrix
body: <p>This is a follow-up question to a previous <a href="https://stackoverflow.com/questions/5176763/r-function-to-create-band-matrix">post</a>.</p>
  
  <p>I need to write an <code>initialize()</code> method for a S4 class (named <code>Band</code>) that expects a matrix and a value for <code>k</code>. It should be defined as a function with formal arguments <code>(x, k)</code>, and should put the lower triangular elements of <code>X</code> into the object.</p>
  
  <p>Any suggestions appreciated.</p>