The l3kernel bundle provides an implementation of the LaTeX3
programmers' interface, as a set of packages that run under
LaTeX2e. The interface provides the foundation on which the
LaTeX3 kernel and other future code are built: it is an API for
TeX programmers. The packages are set up so that the LaTeX3
conventions can be used with regular LaTeX2e packages.

Homepage:
https://ctan.org/pkg/l3kernel
