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8 And Cain talked with Abel his brother: and it came to pass when they were in the field, that Cain rose up against Abel his brother, and slew him.
'To open,' as in Revelation 9, signifies communication and conjunction.
(References: Apocalypse Explained 537)
11 The angel of Yahweh said to her, "Behold, you are with child, and will bear a son. You shall call his name Ishmael, because Yahweh has heard your affliction.