I am reading one book and the author presents what he calls a distributivity formula:...

Gretchen Schwartz
Answered
2022-07-01
I am reading one book and the author presents what he calls a distributivity formula:
where (the set of all sequences {}).
My question is regarding the notation. The LHS is quite clear to me. However, I'm now sure what are actually the members of K. Say and . What is actually here?