Over 20 years ago, I was home on a sick day in the middle of refactoring a build system based on Bourne shell. We could not use any of the new features of BASH

Over 20 years ago, I was home on a sick day in the middle of refactoring a build system based on Bourne shell. We could not use any of the new features of BASH but had to stick to what worked as /bin/sh across a bunch of supercomputer vendors. I was finding a lot of arcane tricks to optimize execution on those systems, some of which had improbably expensive fork-exec. In my fevered state, I wrote some arithmetic functions to do the equivalent of `expr $a + $b` and `expr $a \* $b` without any fork-exec to external commands, just to prove to myself that it could be done. I think I abused things like $IFS, for loops, recursive function calls, and case statements to make an arbitrary-precision decimal adder function.