</tt> can be either a string or a function
 handle. A string value must refer to an another function which exists
 in the union <tt>U</tt>.
 
 A typical case where tie-breaking is useful is evaluation of
 discontinuous MPC feedback laws:
 
 <tt>uopt = U.feval(x, 'primal', 'tiebreak', 'obj')</tt>
 
 Here, if <tt>x</tt> is contained in multiple polyhedra, then the function <tt>primal</tt>
 is only evaluated in the polyhedron which contain <tt>x</tt> and simultaneously has
 the <b>smallest</b> value of the tie