Block diagonal matrix
bd=:[ , ] ,"1~ 0 {.@# [
(2 3$5)bd (4 1$2)
5 5 5 0
5 5 5 0
0 0 0 2
0 0 0 2
0 0 0 2
0 0 0 2
Minor matrices
mm=.1&(|:\.)"2^:2
Returns minor matrices of the order 1 less than the order of original matrix.
Vector cross product
cross=:(1 _1 1 * 1 (-/ . *)\. ])@,.