THROW tag
(THROW tag value)

	command.  Must be used within the scope of a CATCH with an equal
	tag.  Ends the running of the instructionlist of the CATCH.  If
	THROW is used with only one input, the corresponding CATCH does
	not output a value.  If THROW is used with two inputs, the second
	provides an output for the CATCH.

	THROW "TOPLEVEL can be used to terminate all running procedures and
	interactive pauses, and return to the toplevel instruction prompt.
	Typing the system interrupt character (normally control-C for Unix,
	control-Q for DOS, or command-period for Mac) has the same effect.

	THROW "ERROR can be used to generate an error condition.  If the
	error is not caught, it prints a message (THROW "ERROR) with the
	usual indication of where the error (in this case the THROW)
	occurred.  If a second input is used along with a tag of ERROR,
	that second input is used as the text of the error message
	instead of the standard message.  Also, in this case, the location
	indicated for the error will be, not the location of the THROW,
	but the location where the procedure containing the THROW was
	invoked.  This allows user-defined procedures to generate error
	messages as if they were primitives.  Note: in this case the
	corresponding CATCH "ERROR, if any, does not output, since the second
	input to THROW is not considered a return value.

	THROW "SYSTEM immediately leaves Logo, returning to the operating
	system, without printing the usual parting message and without
	deleting any editor temporary file written by EDIT.