Software support for physical quantities

Early in the 20th century, expressions involving abstract physical quantities gained acceptance. For example, Newtons third law, F = ma, holds regardless of units, provided weunderstand and apply the rules of quantity calculus. Scientists, engineers and technicians are trained to use quantity equations. Perhaps unsurprisingly, this skill is difficult to express in computer software. This paper looks at a few ideas that could be useful, some more than a century old