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It is ambiguous whether "0" refers to a nested field name or to a position in the array. For instance, what should the following projection return?
One answer is that "0" should be interpreted as an array position, meaning that we should project just the first array element. The transformed document would then be:
Another answer is that "0" should be interpreted just like any other field name, and should be used to project within each nested document. Under this interpretation, the result is:
Our projection language chooses the latter interpretation.