At least in MKIV. I haven't tried MKII.
The following example should demonstrate this. With no mode
specified on the command line, this should enable mode three and
prevent and disable the other modes. It seems that \preventmode is
not only ineffective in what it is described as doing, but also
disables the following \disablemode!
Or perhaps I am misusing this or misunderstand what it should do.
(2014-08-29 20:57 standalone)
\definemode[one][keep]
\definemode[two][keep]
\definemode[three][keep]
\define\ModeOne{nil}
\define\ModeTwo{nil}
\define\ModeThree{nil}
\startmode[one]
\define\Mode{one}
\define\ModeOne{set}
\disablemode[two,three]
\stopmode
\startmode[two]
\define\Mode{two}
\define\ModeTwo{set}
\disablemode[one,three]
\preventmode[one]
\stopmode
\startnotmode[one,two]
\define\Mode{three}
\define\ModeThree{set}
\enablemode[three]
\preventmode[one,two]
\disablemode[one,two]
\stopnotmode
\starttext
Mode is \Mode.
ModeOne is \ModeOne.
ModeTwo is \ModeTwo.
ModeThree is \ModeThree.
Mode
\doifmode{one}{one}\doifmode{two}{two}\doifmode{three}{three} is
active.
\stoptext
--
Rik Kabel