BOTH allows you to trim or extend two intersecting 
entities (excluding circles). This function is view 
dependent.

Select the first entity to trim or extend, on the side 
that is to remain.

Indicate the second intersecting or potentially
intersecting entity, on the side that is to remain with
respect to where it intersects the first entity to be
trimmed or extended.

The 2-D intersection is computed, and if an intersection
exists, both entities are trimmed or extended to the
projected intersection point.

If the trimming or extending is not correct, press 
(Alt)-(I), or click on Undo in the break area to erase 
the trim or extension.


