code: def find_stoichiometrically_balanced_cycles(model):
      u
      helpers.close_boundaries_sensibly(model)
      fva_result = flux_variability_analysis(model, loopless=False)
      return fva_result.index[
          (fva_result["minimum"] <= (-1 + TOLERANCE_THRESHOLD)) |
          (fva_result["maximum"] >= (1 - TOLERANCE_THRESHOLD))
      ].tolist()
docs: u"""
      Find metabolic reactions in stoichiometrically balanced cycles (SBCs).
  
      Identify forward and reverse cycles by closing all exchanges and using FVA.
  
      Parameters
      ----------
      model : cobra.Model
          The metabolic model under investigation.
  
      Notes
      -----
      "SBCs are artifacts of metabolic reconstructions due to insufficient
      constraints (e.g., thermodynamic constraints and regulatory
      constraints) [1]_." They are defined by internal reactions that carry
      flux in spite of closed exchange reactions.
  
      References
      ----------
      .. [1] Thiele, I., & Palsson, B. Ø. (2010, January). A protocol for
             generating a high-quality genome-scale metabolic reconstruction.
             Nature protocols. Nature Publishing Group.
             http://doi.org/10.1038/nprot.2009.203