An experimental package which implements an environment,
blockarray, that may be used in the same way as the array or
tabular environments of standard LaTeX, or their extended
versions defined in array. If used in math-mode, blockarray
acts like array, otherwise it acts like tabular. The package
implements a new method of defining column types, and also
block and block* environments, for specifying sub-arrays of the
main array. What's more, the \footnote command works inside a
blockarray.

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