[Boost-build] Confused with conditional rule

Subject: [Boost-build] Confused with conditional rule
From: Dmitry Timoshenko (dimentiy2k_at_[hidden])
Date: 2011-09-22 18:20:50

Hello,

Following conditional rule produces an error

used

rule use-complex-xml-properties ( properties * )
{
   if --use-complex-xml in [ modules.peek : ARGV ] ||
     <define>UNICOMM_USE_COMPLEX_XML in $(properties)
   {
     local result = <library>/smart//smart_data ;

     if ! <define>UNICOMM_USE_COMPLEX_XML in $(properties)
     {
# if uncomment this line below the bjam exits with the error
# error: Can not evaluate conditional properties
#
#      result += <define>UNICOMM_USE_COMPLEX_XML ;
     }

     return $(result) ;
   }
}

why when I uncomment the line
'result += <define>UNICOMM_USE_COMPLEX_XML ;'
I get the error?

Please could anybody tell me what am I doing wrong?

Thank you.