more robust reports: ensure that markup is actually present;

tuned signature;

clarified internal tool output: prefer Pretty.pure_string_of over manipulation of print_mode;

unused;

misc tuning and clarification (see also 86a31308a8e1);

tuned signature: more operations;

clarified modules;

dismantle print_mode operations for Markup/Pretty: hardwired check of "print_mode_active Print_Mode.PIDE";

clarified YXML bootstrap;

tuned;