code: def tmpl_ifdef(self, field, trueval=u'', falseval=u''):
          
          if field in self.values:
              return trueval
          else:
              return falseval
docs: If field exists return trueval or the field (default) otherwise,
          emit return falseval (if provided).
  
          * synopsis: ``%ifdef{field}``, ``%ifdef{field,text}`` or \
              ``%ifdef{field,text,falsetext}``
          * description: If field exists, then return truetext or field \
              (default). Otherwise, returns falsetext. The field should be \
              entered without $.
  
          :param field: The name of the field
          :param trueval: The string if the condition is true
          :param falseval: The string if the condition is false
          :return: The string, based on condition