rem: if(!thread_flags) return NULL;
add: if(!thread_flags){ thread_flags = TF_show_proc; return NULL; }  if(forest_type){ return "Thread display conflicts with forest display."; }
context: static const char *thread_option_check(void){  if(!thread_flags) return NULL;  if((thread_flags&TF_B_H) && (thread_flags&(TF_B_m|TF_U_m)))    return "Thread flags conflict; can't use H with m or -m.";  if((thread_flags&TF_B_m) && (thread_flags&TF_U_m))    return "Thread flags conflict; can't use both m and -m.";  if((thread_flags&TF_U_L) && (thread_flags&TF_U_T))    return "Thread flags conflict; can't use both -L and -T.";  return NULL;}