I have trouble understanding this problem:
Verify that the circular arcs (including degenerate ones) of angle less than $\pi$ form a semiring; show that without this additional restriction the assertion is false.
It is an exercise from the book "Real Analysis: Measures, Integrals and Applications".
I tried to write circular arcs like intervals $[a,b]$ where $|b-a|<\pi=$ but it didn't turn out well. Any ideas?