tril
Lower triangular part of a matrix.
` t=tril(a)`
` t=tril(a,k)`
 Inputs `a` Any matrix. `k` An integer. Outputs `t` The lower triangular part of `a` , or the 'pseudo' lower triangular part as defined below.

Description
The output is of the same size as `a` . If `k` is not specified, the output is a copy of the input except that all the elements in the upper triangular part, i.e. the elements of the type `t[i,j]` for `i` less than `j` are replaced by zeros in the output.

If `k` is specified to be positive, all the elements above the `k'th` super diagonal are zero in in the output, i.e., the elements of the type `t[i,j],i<(j-k)` are set to zero.

If `k` is specified to be negative, all the elements below the `(-k)'th` sub diagonal are zero in in the output, i.e., the elements of the type `t[i,j],i<(j-k)` are set to zero.
Example
```>>a=[1 2 3 4;5 6 7 8; 9 10 11 12]
>>a
1          2          3          4
5          6          7          8
9         10         11         12

>>tril(a)
1          0          0          0
5          6          0          0
9         10         11          0

>>tril(a,1)
1          2          0          0
5          6          7          0
9         10         11         12

>>tril(a,2)
1          2          3          0
5          6          7          8
9         10         11         12
>>tril(a,-1)
0          0          0          0
5          0          0          0
9         10          0          0

```