description of third column of em_table:

	-: pop item indicated by next character
	+: push item indicated by next character
	0: no effect on the stack

characters describing items:

	w: target machine word (1, 2 or 4)
	d: double target machine word (2, 4 or 8)
	p: target machine address
	a: item with size specified in argument
	x: item with size specified by top item of stack
	y: item with size specified by second item on stack
	?: one or more items of unknown size
