def sense=(sense)
      @sense = sense
      sense = sense == :min ? GLPK::GLP_MIN : GLPK::GLP_MAX
      GLPK.glp_set_obj_dir @ptr, sense
    end
def sense=(sense)
      @sense = sense
      sense = sense == :min ? GLPK::GLP_MIN : GLPK::GLP_MAX
      GLPK.glp_set_obj_dir @ptr, sense
    end
Set the sense of the model