docs: def flux_bound(self, reaction, direction):
          """Return the flux bound of the reaction.
  
          Direction must be a positive number to obtain the upper bound or a
          negative number to obtain the lower bound. A value of inf or -inf is
          returned if the problem is unbounded.
          """
          try:
              self.maximize({reaction: direction})
          except FluxBalanceError as e:
              if not e.result.unbounded:
                  raise
              return direction * _INF
          else:
              return self.get_flux(reaction)