I have a simple tabulate environment like this:

\starttext

        \starttabulate[|p(.45\textwidth)|cp(.1\textwidth)|p(.45\textwidth)|]            \NR
                \HL
                \NC Cause    \NC \NC Effect    \NC\NR
                \HL
                \NC            \NC \NC            \NC\NR
                 \NC   text a         \NC text b \NC       \input knuth     \NC\NR %<-- vertically center this row
                \HL
            \NR
        \stoptabulate

\stoptext
I'd like to vertically center the content in the one marked row. By default, they seem to place each cell aligned to the top, but how can I adjust them vertically so the text is vertically centered just in specific rows?
--Joel