transaction retries

[Mark Volkmann]

After reading http://clojure.org/refs, it's not clear to me what
situations cause a transaction to retry. Is it when there is an
attempt to change the value of a Ref for which a change has been
committed in another transaction since the current transaction began?
Does this differ based on whether the Ref is changed with ref-set,
commute or alter?

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R. Mark Volkmann
Object Computing, Inc.