Problem 1882. Upper triangular matrix

Create a function to retrieve the upper triangular matrix of a matrix (without using tril). The matrix will always be square.

Example

If a =

     1     2     3     4
     5     6     7     8
     9    10    11    12
    13    14    15    16

then y =

     1     2     3     4
     0     6     7     8
     0     0    11    12
     0     0     0    16