code: def concat_t_vars_np(self, vars_idx=None):
          
          selected_np_vars = self.np_vars
          if vars_idx is not None:
              selected_np_vars = self.np_vars[:, vars_idx]
  
          return np.concatenate([self.np_t[:self.np_nrows].reshape((-1, 1)),
                                 selected_np_vars[:self.np_nrows, :]], axis=1)
docs: Concatenate `self.np_t` with `self.np_vars` and return a single matrix.
          The first column corresponds to time, and the rest of the matrix is the variables.
  
          Returns
          -------
          np.array : concatenated matrix