\yskip\noindent In the latter case, we might have |info(q)=null|. This
is not the same as |math_type(q)=empty|; for example, `\.{\$P\_\{\}\$}'
and `\.{\$P\$}' produce different results (the former will not have the
``italic correction'' added to the width of |P|, but the ``script skip''
will be added).