mathlib

a945b376 - feat(data/sym/basic): add `fill` `filter_ne` and `sigma_ext` (#16316)

feat(data/sym/basic): add `fill` `filter_ne` and `sigma_ext` (#16316) Add definitions and lemmas to support a proof for the multinomial theorem. Co-authored-by: Junyan Xu <junyanxu.math@gmail.com> Co-authored-by: Kyle Miller <kmill31415@gmail.com>