user
Task

Find and display the sum of elements that are below the main diagonal of a matrix.

The matrix should be a square matrix.

:   :   :   ───   Matrix to be used:   ───

```
     [[1,3,7,8,10],
      [2,4,16,14,4],
      [3,1,9,18,11],
      [12,14,17,18,20],
      [7,1,3,9,5]] 

```
assistant
```
sub lower-triangle-sum (@matrix) { sum flat (1..@matrix).map( { @matrix[^$_]»[^($_-1)] } )»[*-1] }

say lower-triangle-sum 
[
    [  1,  3,  7,  8, 10 ],
    [  2,  4, 16, 14,  4 ],
    [  3,  1,  9, 18, 11 ],
    [ 12, 14, 17, 18, 20 ],
    [  7,  1,  3,  9,  5 ]
];

```

Output:

```
69
```