Randomly remove a set number of elements from a list.

Input parameters:

1. Li → List whose items to cull.
2. Rm → Optional number of items to remove. If unspecified, the Retain input must be set.
3. Rt → Optional number of items to retain. If unspecified, the Remove input must be set.
4. Rn → Random engine used for removal.

Output parameters:

1. Li → List without the culled items.
2. Cp → The applied culling pattern.