[Mark T. DeNucci, Sr.]

A1 has not obtain(NFHS)/established(FIBA/NCAA) legal guarding position becuase to do that he had to orginial face B1 when taking his position on the court. But A1 has secured a legal position on the court and therfore is setting a legal screen.

And for all of you who are scratching your heads about a defensive player setting screens, the definition of guarding specifically defines it as something done by the defense. The definition of screening makes no disctinct between offensive or defensive players and specifically notes that the player with the ball can even set a screen.