op-sum[op] seq find the sum of op applied to each of the kets in seq while trying to preserve sequence structure
seq |one> => ssplit |abcde> seq |two> => ssplit |uvw> seq |three> => ssplit |xyz> op-sum[seq] split[" "] |one two three> |a> + |u> + |x> . |b> + |v> + |y> . |c> + |w> + |z> . |d> . |e>