Conex_utils.UintSource64 bit unsigned integer with explicit overflow behaviour (see Uint.succ).
A 64 bit unsigned integer (using a int64).
succ t is carry, next: if carry is true, an overflow happened. next is always the next integer in the Z/2^64-1Z group.
of_string s attempts to parse the string s as hexadecimal encoded number using Int64.of_string "0x" ^ s.