I first met eqn in the mid 1980s. It was amazing that you could write pretty complex math with just a few special literals like “sup” and “sum”, and the braces

I first met eqn in the mid 1980s. It was amazing that you could write pretty complex math with just a few special literals like “sup” and “sum”, and the braces. It turns out that compositionality is so strong in math that it’s most of what you need. This isn’t obvious until you try it! Paired with a LaserWriter (vintage 1986, say), and troff, you could get almost book quality typesetting. Later on, TeX got the details of math much better, but the basic language was the same.