Rough implementation of 64-bit integer subtraction

sub64.reds

Red/System []

int64!: alias struct! [high [integer!] low [integer!]]

sub64: func [
	a [int64!] 
	b [int64!]
	return: [int64!]
	/local 
	    diff [int64!] offset [integer!]
][
	diff: declare int64!
	diff/high: a/high - b/high
	diff/low: a/low - b/low
	offset: diff/low
	if negative? offset [offset: negate offset]
	if a/low < offset [diff/high: diff/high - 1]
	diff
]

a: declare int64!
b: declare int64! 


a/high: 10
a/low:  10
b/high: 3
b/low:  3
c: sub64 a b
print-wide [c/high c/low]

a/high: 10
a/low: 2
b/high: 3
b/low: 6
c: sub64 a b
print-wide [c/high c/low]