=i.4        NB. create an Identity matrix of size 4
1 0 0 0
0 1 0 0
0 0 1 0
0 0 0 1
   Id=: =@i.   NB. define as a verb with a user-defined name
   Id 5        NB. create an Identity matrix of size 5
1 0 0 0 0
0 1 0 0 0
0 0 1 0 0
0 0 0 1 0
0 0 0 0 1