Extract the six faces of a box as rectangular boundary curves.

Input parameters:

1. Bx → Box for face extraction.

Output parameters:

1. x- → Left face of the box.
2. x+ → Right face of the box.
3. y- → Front face of the box.
4. y+ → Back face of the box.
5. z- → Bottom face of the box.
6. z+ → Top face of the box.