Hi Eric,
Here is a toy example to get you started.
, proto::when< proto::assign< proto::plus<proto::terminal<placeholder>, _> , _ >
It looks like this "when" matches everything (at least with my compiler), not only plus (as supposed to), for example it matches multiplication as well: proto::display_expr( var_ * 1 = 2 ); proto::display_expr( Solve()( var_ * 1 = 2 ) ); Output: assign( multiplies( // <<<<< multiplication terminal(var_) , terminal(1) ) , terminal(2) ) assign( terminal(var_) , minus( // <<<<< minus terminal(2) , terminal(1) ) ) I'm using proto from boost 1.37.0, gcc 3.4.6. Thanks, Maxim