rem: string parameter_names(void) const { return("1st_stage_sec,2nd_stage_sec,mid_level_%"); }
add: std::string parameter_names(void) const { return("1st_stage_sec,2nd_stage_sec,mid_level_%"); }
context:   string parameter_names(void) const { return("1st_stage_sec,2nd_stage_sec,mid_level_%"); }