[NTG-pdftex] character protrusion with the first letter in a paragraph

Hi, I have been thinking of the question how to make char protrusion work with the first char of a paragraph. The simplest (and quite robust) solution can be as follows: when considering whether a marginal kern should be inserted, we skip certain kinds of `invisible' node (like adjust node, penalty node and the like). Now to make it work with the first char of a par, we also skip null boxes (box with zero dimen and empty contents) at the left margin. The question is whether such behaviour is ``correct'', and what surprises it can bring in certain situations. Thanh