def _arg(self, line):
        '''singularity doesn't have support for ARG, so instead will issue
           a warning to the console for the user to export the variable
           with SINGULARITY prefixed at build.
 
           Parameters
           ==========
           line: the line from the recipe file to parse for ARG
   
        '''
        line = self._setup('ARG', line)
        bot.warning("ARG is not supported for Singularity! To get %s" %line[0])
        bot.warning("in the container, on host export SINGULARITY_%s" %line[0])