At what point does an arbitrator's exercise of his or her authority to interpret a contract cross a boundary into an impermissible re-writing of that contract?

QUESTION PRESENTED While courts exercise prudential restraint when considering whether to vacate an arbitration award under 9U.S.C.§10(a)(4) (“where arbitrators exceeded their powers ... .”), and while the scope of a court’s review is limited under that statute, this must be balanced with the principle that the scope of an arbitrator’s power in a case is limited by the terms of the parties’ agreement to arbitrate, and courts must correct arbitration awards that exceed the scope of the authority granted to the arbitrator. Pursuant to these competing concepts: At what point does an arbitrator’s exercise of his or her authority to interpret a contract cross a boundary into an impermissible re-writing of that contract?