A compound of formula I ** (See formula) ** or a pharmaceutically acceptable salt thereof, in which: X is -C (= O) NHR4; R1, R2, R3 are independently H or F; R4 is H, C1-6 alkyl, or O-C1-6 alkyl; each dotted line ... is independently an optional single link; and each time it appears, C1-6 alkyl is optionally substituted with 1-3 halo, OH, O-C1-4 alkyl, CN, N3, O (C = O) C1-4 alkyl, NH2, NHC (= O ) C1-4 alkyl, or C (= O) C1-4 alkyl.
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