AMSTeX is a TeX macro package, originally written by Michael
Spivak for the American Mathematical Society (AMS) during 1983-
1985 and is described in the book 'The Joy of TeX'. It is based
on Plain TeX, and provides many features for producing more
professional-looking maths formulas with less burden on
authors. More recently, the focus of attention has switched to
amslatex, but AMSTeX remains as a working system.

Homepage:
https://ctan.org/pkg/amstex
