feat(algebra/direct_sum/basic): some lemmas about `direct_sum.of` (#10003)
feat(algebra/direct_sum/basic): some lemmas about `direct_sum.of` (#10003)

Some small lemmas about `direct_sum.of` that are handy.



Co-authored-by: jjaassoonn <4458de33c6b1a8c5e93ff2e86b4b4ba478e32623@out.com>

leanprover-community/mathlib,leanprover-community/mathlib