AutoLISP Help – Keep Only Outermost Polyline Inside Boundary

[Tamim]

Description:
I have a sample drawing where:

The outer boundary is in red color (closed polyline).

Inside this boundary, there are several cross polylines (in white).

Some of these inside polylines are offsets or overlapping.

My Requirement:

I want to keep only the outermost polyline inside the boundary.

The inner offset/cross polylines should be trimmed or deleted automatically.

The result should leave only the clean outer boundary and remove the unwanted inner lines.

Sample.dwg