[NTG-context] programming: doifdefined & friends

While I can't cope with counters, I'm trying other stuff that I don't understand: How must I use \doifdefined & friends? Consider this snippet: ----- \starttext \def\myvar{JAJAJA} \input tufte \doifdefined{\myvar}{\par bla \myvar blub\par} \input tufte \stoptext ----- I'd guess it would print "bla JAJAJAblub" between the two Tufte texts and don't if I comment \def\myvar. But I guess wrongly: It does nothing in this case, and complains about undefined \myvar in the latter. But if I really want to check if something is defined before I try to use it, how must I write it? (Expansion magic?) Greetlings from Lake Constance! Hraban --- http://www.fiee.net/texnique/ http://contextgarden.net http://www.cacert.org (I'm an assurer)