Proves what the cutoff difference of two natural number numerals is.
DESCRIPTION
If n and m are numerals (e.g. 0, 1, 2, 3,...), then
NUM_SUB_CONV `n - m` returns the theorem:
|- n - m = s
where s is the numeral that denotes the result of subtracting the
natural number denoted by m from the one denoted by n, returning zero for
all cases where m is greater than n (cutoff subtraction over the natural
numbers).
FAILURE CONDITIONS
NUM_SUB_CONV tm fails if tm is not of the form `n - m`, where n and
m are numerals.
EXAMPLE
# NUM_SUB_CONV `4321 - 1234`;;
val it : thm = |- 4321 - 1234 = 3087
# NUM_SUB_CONV `77 - 88`;;
val it : thm = |- 77 - 88 = 0
COMMENTS
Note that subtraction over type :num is defined as this cutoff subtraction.
If you want a number system with negative numbers, use :int or :real.