I would like to make a "row" of a \startalign ... \stopalign "hidden". The following fails.

\starttext
    \definelayer[hiddenlayer][state=stop]
    \startformula
        \startalign[n=3,align={right,middle,left}]
            \NC a \NC = \NC b \NR
            \setlayer[hiddenlayer]{\NC a + c \NC = \NC b + c \NR}
        \stopalign
    \stopformula
\stoptext

Troy Henderson