\usemodule[filter]
        \defineexternalfilter[rest][filtercommand={pandoc -f rst -t context -o 
\externalfilteroutputfile\space \externalfilterinputfile}]
\starttext
\startrest
=====	======
first	second
=====	======
 a_b	1.6
$a_b$	2.5
=====	======
\stoprest
\stoptext