\def\beq{\begin {equation}}
\def\eeq{\end {equation}}
\def\beqa{\begin {eqnarray}}
\def\eeqa{\end {eqnarray}}


\def\data{\bf d}
\def\model{\bf m}
\def\zero{\bf 0}
\def\xvar{\bf x}
\def\pvar{\bf p}
\def\lop{\bf L}
\def\zero{\bf 0}
\def\iop{\bf I}
\def\reg{\bf A}
\def\prec{\bf A^{-1}}
\def\time{\bf t}
\def\tomo#1{\bf T_{#1}}
\def\slow#1{\bf s_{#1}}
\def\ds{\bf \Delta s}
\def\dt{\bf \Delta t}
\def\pox{&\approx&}
