title: Sparse Iteration in Chapel
body: <p>Given an array <code>M:[sps]</code> over a <code>sps = sparse subdomain(dom)</code> where <code>dom: domain(n)</code>, I know I can iterate densely over the k​th​​ dimension with <code>for j in M.domain.dim(k) {...}</code>. How would I define an analogous loop that visits only populated locations in k such that if <code>M.domain.dim(k) = {1..N}</code>, but there are only m populated indices in k, the loop only formally generates m iterations rather than N?</p>
  
  <p>Would the way to do this be to iterate over the whole sparse domain with an iterand that only references one index? (i.e. <code>for (i,j) in spsDom { var idx = i }</code>)</p>