Pgs. 58 and 59 of David Markson’s copy of The Complete Greek Drama: Volume Two by Various (Ed. Whitney J. Oates & Eugene O’Neill, Jr.):

     On which Markson has underlined two passages from Euripides’ Helen:
     The first:
     “Withhold then the malignant blade from thy sister, and believe that she herein is acting with discretion.”
     The second (the final lines of the play):
     “Many are the forms the heavenly will assumes; and many a thing God brings to pass contrary to expectation: that which was looked for is not accomplished, while Heaven finds out a way for what we never hoped; e’en such has been the issue here.”

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     As it says in the note on pg. 59 in the above scan, the final lines of Helen are “found likewise at the conclusion of the Alcestis, Andromache, The Bacchae, and, with a slight addition, the Medea.”

     Markson noted this on pg. 67 in The Last Novel:
     “Andromache. Alcestis. Helen. Medea. The Bacchae.
     Each of which Euripides ends with his chorus speaking an identical verse—to the effect that the ways of the gods are unpredictable.”