\starttext

\enablemode[one]
\enablemode[two]

\definemode[one]  [keep]
\definemode[two]
\definemode[three][yes]
\definemode[four]

\startlines
one   : \doifmodeelse{one}  {yes}{nop} == yes
two   : \doifmodeelse{two}  {yes}{nop} == nop
three : \doifmodeelse{three}{yes}{nop} == yes
four  : \doifmodeelse{four} {yes}{nop} == nop
\stoplines

\stoptext
