objective:

• std::tuple traversal functions

dependencies:

• std::tuple

usage:

• the last argument is the functor handling the elements
• the tuples can be passed by left reference, right reference or const reference
• '_if' versions cancel traversal if the functor returns false

unary:

• traverse::elements( tuple< a, b, c >(), f );
  => f( a ); f( b ); f( c );
• traverse::combinations( tuple< a, b, c >(), f );
  => f( a, b ); f( a, c ); f( b, c );
• traverse::pairs( tuple< a, b, c >(), f );
  => f( a ); f( a, b ); f( b, c );
• traverse::reduce( tuple< a, b, c >(), f );
  => f( f( a, b ), c );

binary:

• traverse::parallel( tuple< a, b, c >(), tuple< d, e >(), f );
  => f( a, d ); f( b, e );
• traverse::combinations( tuple< a, b, c >(), tuple< d, e >(), f );
  => f( a, d ); f( a, e ); f( b, d ); f( b, e ); f( c, d ); f( c, e );