I was looking at Cold Duck Time :
Where the key signature is F major. On the second bar, the bass line says that the A is to be played natural. The key signature doesn't have Ab, but the chord (Bb7) that is being played at that bar has; does this chord imply that if it weren't for the natural sign, the A was to be flat?
How am I supposed to play the A at the 4th bar? Natural or Flat?