!4#2400 Analysis.Normed.Group.Seminorm

Stream: mathlib4

[Jireh Loreaux]

We've finally made it to analysis! :tada: This is the first file in the analysis folder and it's ready for review. It has two dependent PRs (!4#2401 to forward port some necessary material from mathlib3 and !4#2419 for a fix to to_additive).