6-substituted 1,2,4a,5a,8a,8b-hexahydro- and 1,2,3,4,4a,5a,8a,8b-octahydro-6H-pyrrolo[3',4': 4,5]furo[3,2-b]pyrid-8(7H)-one derivatives and their use in combatting endoparasites

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