NAME
asn_sub - subtract one arbitrary size number from another
SYNOPSIS
string asn_sub(string a, string b, string m)
DESCRIPTION
Compute a minus b modulo m. The modulus must be larger than zero.
Arbitrary size numbers are encoded as strings, most significant byte
first. The most significant bit in the first byte, when set, indicates
that the number is negative and encoded in two's complement.
ERRORS
An error will result if the modulus is less than or equal to zero.