Hi,
Not having too many ideas, I did this: is there not a more elegant way ?
Thank you.
Fabrice

\definemathmatrix [pmatrix][matrix:parentheses][simplecommand=MATRIX]
\starttext
\startmidaligned
            \startxtable[align={middle,lohi},frame=off,offset=0.8ex]
              \startxrow
                \startxcell
                  \m{P\times M=P}
                \stopxcell
                \startxcell
                  \math{\iff}
                \stopxcell
                \startxcell
                  \m{\MATRIX{x,y}\times
\MATRIX{0.22,0.78;0.53,0.47}=\MATRIX{x,y}}
                \stopxcell
              \stopxrow
              \startxrow
                \startxcell

                \stopxcell
                \startxcell
                  \m{\iff}
                \stopxcell
                \startxcell[align=right]
                  \math{\startmathcases
                         \NC 0.22x+0.53y=x    \NR
                         \NC 0.78x+0.47y=y     \NR
                         \stopmathcases}
                \stopxcell
              \stopxrow
              \startxrow
                \startxcell

                \stopxcell
                \startxcell
                  \m{\iff}
                \stopxcell
                \startxcell[align=right]
                  \m{0.78x-0.53y=0}
                \stopxcell
              \stopxrow
              \startxrow
                \startxcell

                \stopxcell
                \startxcell
                  \m{\iff}
                \stopxcell
                \startxcell[align=right]
                 \m{78x-53y=0}
                \stopxcell
              \stopxrow
            \stopxtable
          \stopmidaligned
\stoptext