  For each function on bit strings, its restriction to bit strings of any given
length can be computed by a finite instruction sequence that contains only
instructions to set and get the content of Boolean registers, forward jump
instructions, and a termination instruction. Backward jump instructions are not
necessary for this, but instruction sequences can be significantly shorter with
them. We take the function on bit strings that models the multiplication of
natural numbers on their representation in the binary number system to
demonstrate this by means of a concrete example. The example is reason to
discuss points concerning the halting problem and the concept of an algorithm.
