global constant DEBUG = 0  -- (or any other identifier name can be used)
global procedure assert(integer flag, string msg)
    if DEBUG then
        if not flag then
            {} = message_box(msg,"failed assertion",MB_OK)  -- or
            puts(1,msg)                                     -- , and/or
            crash(msg)      -- crash/ex.err report          -- or
            trace(1)        -- start debugging
        end if
    end if
end function

assert(i=42,"i is not 42!!")