docs: def transpile_modname_source_target(self, spec, modname, source, target):
          """
          The function that gets called by compile_transpile_entry for
          processing the provided JavaScript source file provided by some
          Python package through the transpiler instance.
          """
  
          if not isinstance(self.transpiler, BaseUnparser):
              _deprecation_warning(
                  'transpiler callable assigned to %r must be an instance of '
                  'calmjs.parse.unparsers.base.BaseUnparser by calmjs-4.0.0; '
                  'if the original transpile behavior is to be retained, the '
                  'subclass may instead override this method to call '
                  '`simple_transpile_modname_source_target` directly, as '
                  'this fallback behavior will be removed by calmjs-4.0.0' % (
                      self,
                  )
              )
              return self.simple_transpile_modname_source_target(
                  spec, modname, source, target)
  
          # do the new thing here.
          return self._transpile_modname_source_target(
              spec, modname, source, target)