The Flatten(L) function recursively converts any elements in a list which are of type list to expression sequences.

The Flatten(L, N) function flattens a nested list of lists $N$ times.

Note: The inlined procedure $\mathrm{FlattenOnce}\left(L\right)$ is more efficient than a call to Flatten(L, 1).

$\mathrm{with}\left(\mathrm{ListTools}\right)\:$

$L≔\left[1\,2\,3\,\left[4\,5\,6\right]\right]$

$L≔\left[1\,2\,3\,\left[4\,5\,6\right]\right]$

$\mathrm{Flatten}\left(L\right)$

$\left[1\,2\,3\,4\,5\,6\right]$

$L≔\left[1\,\left[2\right]\,\left[3\,\left[4\right]\right]\,\left[\left[\left[5\right]\right]\,6\right]\right]$

$L≔\left[1\,\left[2\right]\,\left[3\,\left[4\right]\right]\,\left[\left[\left[5\right]\right]\,6\right]\right]$

$\mathrm{Flatten}\left(L\right)$

$\left[1\,2\,3\,4\,5\,6\right]$

$\mathrm{Flatten}\left(L\,1\right)$

$\left[1\,2\,3\,\left[4\right]\,\left[\left[5\right]\right]\,6\right]$