title: How to build a multimap from a list of tuples in Scala?
body: <p>Suppose I have a list of tuples <code>List[(A, B)]</code>. What is the best way to convert it to a <code>multimap</code>, which maps <code>A</code> to <code>Set[B]</code>? Can I build an <em>immutable</em> <code>multimap</code> ?</p>