I have written an autolisp file to trim arcs from the center/insertion point of a circle block. how it works
select first block.
select second block
arc is made from insertion point of first block
user picks midpoint of arc.
arc ends at the insertion point of the 2nd block
then it trims the arc inside the block (circle) from the insertion point.

my only issue is if the windows is not where the point for the trim is then it will show a hatched windows to select the next point. (this is the part has to be fixed. im not doing anything fancy. just using this for the trim.

Code:
(command "trim" square "" npnt ""))