Priority arguments and separation problems

Different constructions in the recursion theory use the so-called priority arguments. A general scheme was suggested by A.~Lachlan. Based on his work, we define the notion of a priority-closed class of requirements. Then, for a specific priority construction, we need to check only that all requirements we want to satisfy belong to some priority-closed class (defined in game terms). This game version of Lachlan's approach is used to present some results about recursively inseparable sets obtained by the author.