code: def format_sass_stack(self):
          
          if not self.rule_stack:
              return ""
  
          ret = ["on ", self.format_file_and_line(self.rule_stack[0]), "\n"]
          last_file = self.rule_stack[0].source_file
  
          # TODO this could go away if rules knew their import chains...
          # TODO this doesn't mention mixins or function calls.  really need to
          # track the call stack better.  atm we skip other calls in the same
          # file because most of them are just nesting, but they might not be!
          # TODO the line number is wrong here for @imports, because we don't
          # have access to the UnparsedBlock representing the import!
          # TODO @content is completely broken; it's basically textual inclusion
          for rule in self.rule_stack[1:]:
              if rule.source_file is not last_file:
                  ret.extend((
                      "imported from ", self.format_file_and_line(rule), "\n"))
              last_file = rule.source_file
  
          return "".join(ret)
docs: Return a "traceback" of Sass imports.