docs: def update(self, dt=-1):
          """
          Return type string, compatible with numpy.
          """
          self.library.update.argtypes = [c_double]
          self.library.update.restype = c_int
          if dt == -1:
              # use default timestep
              dt = self.get_time_step()
          result = wrap(self.library.update)(dt)
          return result