intertext for aligned equations

[Aditya Mahajan <adityam@umich.edu>]

Hi,
  I was looking at trying to implement something similar to 
amsmath's intertext command. I can get the basic functionality working 
as follows.

\def\startintertext#1\stopintertext{%
     \noalign{#1}\NR}

\startformula \startalign
   \NC A_1 \NC= \left| \int_0^1 (x^2 - 3x)\, dx \right|
     + \left| \int_1^2 (x^2 -5x + 6)\, dx \right| \NR
   \startintertext
     Take limis of both integrals %\input tufte
   \stopintertext
   \NC \NC = \left| \frac{x^3}{3} - \frac{3}{2} x^2 \right|_0^1
     + \left| \frac {x ^3}{3} - 7 \frac{5}{2} x^2 + 6x \right|_1^2 \NR
     \NC \NC = \left| - \frac{7}{6} \right| + \left|\frac{14}{3} -
     \frac{23}{6}\right| = \frac{7}{6} + \frac{5}{6} = 2 \NR
\stopalign \stopformula

This seems to be working fine. However, \intertext defined in 
amsmath.sty seems to be doing  a lot more (I just can't figure out 
what it is doing?) Can someone suggest how to make the above command 
more robust on the lines of \intertext?

%----- from amsmath.sty -------------
\def\intertext@{%
   \def\intertext##1{%
     \ifvmode\else\\\@empty\fi
     \noalign{%
       \penalty\postdisplaypenalty\vskip\belowdisplayskip
       \vbox{\normalbaselines
         \ifdim\linewidth=\columnwidth
         \else \parshape\@ne \@totalleftmargin \linewidth
         \fi
         \noindent##1\par}%
       \penalty\predisplaypenalty\vskip\abovedisplayskip%
     }%
}}

Thanks,
Aditya

-- 
Aditya Mahajan, EECS Systems, University of Michigan
http://www.eecs.umich.edu/~adityam || Ph: 7342624008