 conditions <tt>bi</tt>.  In particular the <tt>bi</tt> may be equations,
  lists of equations (such as that returned by <tt>solve</tt>), or
  assignments, in which cases <tt>ev</tt> "plugs" the <tt>bi</tt> into
  <tt>a</tt>.  The <tt>Bi</tt> may also be words such as <tt>numer</tt> (in
  which case the result is returned in numerical format), <tt>detout</tt>
  (in which case any matrix inverses in <tt>a</tt> are performed with the
  determinant factored out), or <tt>diff</tt> (in which case all
  different