`mix` and `split` are not inverses of each other

[fzipp]

mix and split are supposed to be each other's inverses, but currently they are not:

A = 3 3 3 rho iota 9; A
	1 2 3
	4 5 6
	7 8 9
	
	1 2 3
	4 5 6
	7 8 9
	
	1 2 3
	4 5 6
	7 8 9
split A
	(1 2 3 4 5 6 7 8 9) (1 2 3 4 5 6 7 8 9) (1 2 3 4 5 6 7 8 9)
mix split A
	1 2 3 4 5 6 7 8 9
	1 2 3 4 5 6 7 8 9
	1 2 3 4 5 6 7 8 9