code: def _check_L(self, L):
          
          # TODO: Take this out?
          if issparse(L):
              L = L.todense()
  
          # Check for correct values, e.g. warning if in {-1,0,1}
          if np.any(L < 0):
              raise ValueError("L must have values in {0,1,...,k}.")
docs: Run some basic checks on L.