def write_mps(filename)
      ret = GLPK.glp_write_mps @ptr, GLPK::GLP_MPS_FILE, nil, filename
      fail if ret != 0
    end
def write_mps(filename)
      ret = GLPK.glp_write_mps @ptr, GLPK::GLP_MPS_FILE, nil, filename
      fail if ret != 0
    end
Write the model to a file in MPS format