Ero cras - "To-morrow I will be [here]" - is the phrase resulting from taking the first letter of the second word in each of the O Antiphons in reverse order (Emmanuel, Rex, Oriens, Clavis, Radix, Adonai, Sapientia): for they foretell the imminent arrival of Our Saviour. Happy coincidence? Probably. It's just the sort of delightful ex post facto explanation and signification that the medieval writers loved to find.
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