Pringsheim Notation for continued fractions

Dear All, 


I would like to typeset continued fractions as attempted and failed in the second line of the following MWE:

\def\cFrac#1#2{%
\vcenter{\hbox{\strut$#1$\,\vrule}\hrule\hbox{\strut\vrule\,$#2$}}}

\starttext
\startformula
\startalign
\NC  a_0+a_0a_1+a_0 a_1 a_2+a_0 a_1 a_2 a_3 \NC  = \cfrac{a_0}{1-\cfrac{a_1}{1+a_1-\cfrac{a_2}{1+a_2-\cfrac{a_2}{1+a_2-\cfrac{a_3}{1+a_3-\cdots}}}}}\NR
%\NC   \NC  = \frac{a_0|}{|1}-\frac{a_1|}{|1+a_1}-\frac{a_2|}{|1+a_2}-\frac{a_3|}{|1+a_3}-\cdots\NR
\NC   \NC  = \cFrac{a_0}{1}-\cFrac{a_1}{1+a_1}-\cFrac{a_2}{1+a_2}-\cFrac{a_3}{1+a_3}-\cdots\NR
\stopalign

\stopformula
\stoptext


The macro is from 

https://tex.stackexchange.com/questions/68190/how-do-i-typeset-continued-fractions-in-pringsheims-concise-notation

where this question is discussed for LaTeX.

Unfortunately, the vertical bars on the top of the fractions do not align with the right end of the fraction bar.


Any suggestions?

Thanks,

Matthias

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