docs: def subproductPath(self, ext):
          """Makes a subproduct filename by appending 'ext' to the subproduct directory.
          Returns a (filename,fullpath) tuple."""
          ext = ext.lstrip("-")
          basename = os.path.join(os.path.basename(self.dp.subproduct_dir()), ext)
          fullpath = os.path.join(self.dp.subproduct_dir(), ext)
          return basename, fullpath