On or upon + gerund
Solution 1:
Google NGrams states that on+gerund is still used more often than upon+gerund (tried with hearing below, opening and closing - the diagrams were a bit different, but the overall result was the same).
Google NGrams states that on+gerund is still used more often than upon+gerund (tried with hearing below, opening and closing - the diagrams were a bit different, but the overall result was the same).