title: How do I find out if a tuple contains a type?
body: <p>Suppose I want to create a compile-time heterogenous container of <em>unique</em> types from some sequence of non-unique types. In order to do this I need to iterate over the source type (some kind of <code>tuple</code>) and check whether each type already exists in my "unique" tuple.</p>
  
  <p>My question is: <strong>How can I check whether a tuple (or a <code>boost::fusion</code> container) contains a type?</strong></p>
  
  <p>I'm open to using either the STL or <code>boost</code>.</p>