Causal Mask

Build the causal (look-ahead) mask for a sequence of length n: a boolean $n\times nn\backslash times\; n$ matrix whose entry $(i,j)(i,j)$ is True when position $ii$ may attend to position $jj$, i.e. when $j\le ij\backslash le\; i$.

Implement solve(n).

Returns. Return a lower-triangular boolean NumPy array (diagonal included).

Examples

Example 1

Input:

solve(4)

Output:

[[True, False, False, False], [True, True, False, False], [True, True, True, False], [True, True, True, True]]

Example 2

Input:

solve(1)

Output:

[[True]]