within an ace of
  離…只差一點兒
  Ace n.; pl. Aces
  1. A unit; a single point or spot on a card or die; the card or die so marked; as, the ace of diamonds.
  2. Hence: A very small quantity or degree; a particle; an atom; a jot.
     I 'll not wag an ace further.   --Dryden.
  3. Sport A single point won by a stroke, as in handball, rackets, etc.; in tennis, frequently, a point won by a service stroke.
  To bate an ace, to make the least abatement. [Obs.]
  Within an ace of, very near; on the point of.
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