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chore(analysis/normed_space/inner_product): euclidean space is a real inner product space
Oh, I guess [Π i, inner_product_space ℝ (f i)] [Π i, inner_product_space 𝕜 (f i)] is illegal because it puts two different additive group structures on f i...
I don't remember why I wanted this. I remember that I didn't use these instances in the final version, then abandoned this pr. We can close it if it leads to diamonds.