math/sage, also known as SageMath is a very important application for people working in mathematics or sciences.
Unfortunately, this must-have is also a nightmare for packagers, and ATM the FreeBSD port is lagging and broken.
This patch fixes it and upgrades it to the latest stable version (9.1).
This is a kind of meta-application, which builds as many math-related packages as possible to give them a common interface, and all these sub-packages are bundled. Upstream knows about this problem, and a ticket has been opened to "use as many system packages as possible": see #27330 at https://trac.sagemath.org/ticket/27330.
The proposed patch uses system packages from this list, completed with others which are not stage-clean when bundled, and others common ports to avoid as many conflicts as possible.