PL_ACTION_TRACE	Start Prolog tracer
PL_ACTION_DEBUG	Switch on Prolog debug mode
PL_ACTION_BACKTRACE	Print backtrace on current output stream. The argument (an int) is the number of frames printed.
PL_ACTION_HALT	Halt Prolog execution. This action should be called rather than Unix exit() to give Prolog the opportunity to clean up. This call does not return.
PL_ACTION_ABORT	Generate a Prolog abort. This call does not return.
PL_ACTION_BREAK	Create a standard Prolog break environment. Returns after the user types control-D.
PL_ACTION_SYMBOLFILE	The argument (a char *) is considered to be hold the symbolfile for further incremental loading. Should be called by user applications that perform incremental loading as well and want to inform Prolog of the new symbol table.