docs: def keep_sources(self, keep):
          """Keep only the specified sources in the decomposition.
          """
          if self.unmixing_ is None or self.mixing_ is None:
              raise RuntimeError("No sources available (run do_mvarica first)")
          n_sources = self.mixing_.shape[0]
          self.remove_sources(np.setdiff1d(np.arange(n_sources), np.array(keep)))
          return self