Proves what the cutoff predecessor of a natural number numeral is.
DESCRIPTION
If n is a numeral (e.g. 0, 1, 2, 3,...), then
NUM_PRE_CONV `PRE n` returns the theorem:
|- PRE n = s
where s is the numeral that denotes the cutoff predecessor of the
natural number denoted by n (that is, the result of subtracting 1 from it, or
zero if it is already zero).
FAILURE CONDITIONS
NUM_PRE_CONV tm fails if tm is not of the form `PRE n`, where n is a
numeral.
EXAMPLE
# NUM_PRE_CONV `PRE 0`;;
val it : thm = |- PRE 0 = 0
# NUM_PRE_CONV `PRE 12345`;;
val it : thm = |- PRE 12345 = 12344