[SR-Users] Check for existence of URI param

Greetings, I have a Contact URI with params as in the following form returned to me in a 302 Moved Temporarily reply: <sip:a=foo;b;c=bar> I am able to check the existence of parameters 'a' and 'b', which have values assigned to them, in the following way: failure_route[...] { ... $var(dest) = $T_rpl($ct); $var(dest) = $(var(dest){nameaddr.uri}); $var(dest) = $(var(dest){uri.user}); ... if(! $(var(dest){param.value,a})) { .... } ... } However, what I cannot seem to figure out is a reliable test condition for the existence of a parameter like 'b', which does not have a value. I have tried: if(! $(var(dest){param.value,b})) which clearly cannot work as the parameter has no _value_. Also have tried: if(!defined $(var(dest){param.value,b})) and also tests of strempty(), !defined, etc. for $var(dest){uri.param,b}) (prior to extraction of param list using {uri.user} above). Ideas appreciated, thanks! -- Alex Balashov - Principal Evariste Systems LLC 1170 Peachtree Street 12th Floor, Suite 1200 Atlanta, GA 30309 Tel: +1-678-954-0670 Fax: +1-404-961-1892 Web: http://www.evaristesys.com/