Update prob.md

Author: jaspwr

competition/midi/prob.md

@@ -42,7 +42,8 @@ For example, in the following, only at most **1** note is being played at any on
 
 Note that:
 * If a note is turned on again while it's already held or turned off while it's already released, that message does nothing.
-* If a note off and on event are sent on the same frame, those notes do not overlap. The note off events are processed first.
+* If a note off and on event are sent on the same frame (instant in milliseconds), those notes do not overlap. The note off events are processed first.
+* In a frame, if there are the same number of OFF (`0x8c`) instructions and ON (`0x9c`) instructions, then for that frame, the number of notes playing is considered not to be changed.
 
 ## Output