> Hans Aberg <mailto:haberg-1@telia.com>
> 7. Juli 2015 21:11
>> On 7 Jul 2015, at 20:17, Wolfgang Schuster<schuster.wolfgang@gmail.com>  wrote:
>
>> You can set the alignment for each column in a matrix with the align key.
>
> FYI, the document amshelp.pdf says that AMS-LaTeX environments align, align*, and aligned, can have multiple alignments, and gives this example:
> \begin{align*}
>    K&\approx G * H&    i&= j+k&   B&\subset C\\
>    H&\approx A_{0}*B_{0}&   i’&= j’+k’&   C&= D\cap E\\
>    G&\approx \coprod_{\alpha\in A} L_{\alpha}&   i’’&=j’’+k’’&  A&= D \cup E
> \end{align*}
> which is typeset so that 'K&\approx G * H’ is one equation aligned on the ‘&’ with the others below, followed by 'i&= j+k’ in the next column.
>
> So the ampersands have multiple functions here, it seems: alignment and separating columns.
>
> Translating into ConTeXt, one might have (pseudocode):
> \startformula
>    \startalign
>      \NC K \NA \approx G * H \NC  i\NA= j+k \NC B \NA \subset C \NR
>      \NC H \NA \approx A_{0}*B_{0} \NC   i’\NA= j’+k’ \NC   C \NA = D\cap E\NR
>      \NC G \NA \approx \coprod_{\alpha\in A} L_{\alpha}
>        \NC   i’’\NA =j’’+k’’ \NC  A \NA = D \cup E \NR
>    \stopalign
> \stopformula
> where I have introduce a symbol \NA for alignment.
>
You should google for mathalign.pdf which explains and shows examples 
for the matrix, cases etc. environments.

To get the same result as your LaTeX example you can use the m key for 
\startalign.

\starttext

\startformula
     \startalign[m=3,distance=3em]
     \NC K \NC \approx G * H                            \NC  i   \NC = 
j+k     \NC B \NC \subset C  \NR
     \NC H \NC \approx A_{0}*B_{0}                      \NC  i'  \NC = 
j'+k'   \NC C \NC = D\cap E  \NR
     \NC G \NC \approx \coprod_{\alpha\in A} L_{\alpha} \NC  i'' \NC  
=j''+k'' \NC A \NC = D \cup E \NR
     \stopalign
\stopformula

\stoptext

Wolfgang