% \begin{macro}{\addboxcontents}
% Given a vbox |#1|, add to the current vertical list such that the end
% result is equivalent to the list that \TeX\ would have built had the
% contents of |#1| (apart from any initial glue) been added individually
% to the current list.
% So essentially, the problem is that of unboxing |#1|, but replacing
% the glue at the top of |#1| with (something equivalent to) the
% |\baselineskip| or |\lineskip| glue that \TeX\ would normally have
% placed before the first box in |#1|. Also |\prevdepth| must be set at