This calculator showed the feasibility of numerical calculations expressed in an alternate markup. Its derived from an earlier awk script now in coffeescript. github
I wrote this calculator when I volunteered to work out the expenses for our 1981 ski trip to Jackson Hole. I wasn't even sure how to approach the calculation so I just started writing facts into a file.
Pretty soon I realized that if I could just sum the groups and then bring those sums into other groups then I'd pretty much have it. I wrote a small AWK script to do just that and print a report. I finished the script and the report together. webpage.