Interrupts

	While a calculation is in progress, you can generate the SIGINT
	signal, and the calculator will catch it.  At appropriate points
	within a calculation, the calculator will check that the signal
	has been given, and will abort the calculation cleanly.  If the
	calculator is in the middle of a large calculation, it might be
	a while before the interrupt has an effect.

	You can generate the SIGINT signal multiple times if necessary,
	and each time the calculator will abort the calculation at a more
	risky place within the calculation.  Each new interrupt prints a
	message of the form:

		[Abort level n]

	where n ranges from 1 to 3.  For n equal to 1, the calculator will
	abort calculations at the next statement boundary.  For n equal to 2,
	the calculator will abort calculations at the next opcode boundary.
	For n equal to 3, the calculator will abort calculations at the next
	lowest level arithmetic operation boundary.

	If a final interrupt is given when n is 3, the calculator will
	immediately abort the current calculation and longjmp back to the
	top level command level.  Doing this may result in corrupted data
	structures and unpredictable future behavior, and so should only
	be done as a last resort.  You are advised to quit the calculator
	after this has been done.
