"Let be a finite measure on the Borel sigma-algebra of a metric space .
Prove that the class of all Borel sets that are both inner and outer regular is a sigma algebra. Deduce that every Borel set is inner regular."
I have a hard time visualizing what the complement of the inner/outer regular set is and how it all fits together. Any help is appreciated. (I think this is #12 in chapter 2 or Pollard's prob text)