nurbs are converted to arcs, then loaded into the trajectory planner as a x number of arcs.

Function to load arcs in trajectory planner is done by component tpmod.so, this component is also used for jogging :
	tpAddCircle(){}

Lcnc motion component motmod.so :

~linuxcnc/src/emc/motion/command.c line 1088 loads the arc by :
 	tpAddCircle(&emcmotIn......
The function : 	
 	void emcmotCommandHandler_locked(void *arg, long servo_period); line 386
Called by : 
	void emcmotCommandHandler(void *arg, long servo_period); line 1904
Is hal function : 
    retval = hal_export_funct("motion-command-handler", emcmotCommandHandler, 0	/* arg Line 1021

The data for the arcs is stored in a c structure, saved in shared memory : 
	extern struct emcmot_command_t *emcmotCommand;
	- cmd_code_t command;	/* command code (enum) */
		- EMCMOT_SET_LINE,	/* queue up a linear move */
		- EMCMOT_SET_CIRCLE,	/* queue up a circular move */


The interpreter :
interp_queue.hh looks like to store a vector of gcode lines.

milltask.so
/home/user/linuxcnc/src/emc/task/taskintf.cc line 1407
This function sends a arc move to hal component motmod.so
	int emcTrajCircularMove(EmcPose end, PM_CARTESIAN center .....
	
/home/user/linuxcnc/src/emc/task/emctask.cc line 565
Reads from the interpreter directly.
	int emcTaskPlanRead()
	
	Line 46, it has interpreter storage:
	InterpBase *pinterp=0;
	#define interp (*pinterp)
	
liblinuxcnc.so
/home/user/linuxcnc/src/emc/nml_intf/emc.hh
This functions calls the milltask.so 
	extern int emcTrajCircularMove(EmcPose end, ....
	
/home/user/linuxcnc/src/emc/task/emccanon.cc	
This function is called by the interpreter and adds a line to the interp_list by calling tag_and_send(...
	void STRAIGHT_TRAVERSE(....
	
Conclusion.

The interpreter itself has not the function STRAIGHT_TRAVERSE(); itself. Its been called by outside programs that include's the
interpreter source code and therefore must declare the STRAIGHT_TRAVERSE() function.