In a helpmate in three (or more) moves, in the first phase, some piece visits in succession three squares. In another phase, anothe piece of the same color (or the same piece) visits the same three squares also in succession, but in opposite order.
In a helpmate in two moves, reduced form of the theme is allowed: in one phase some piece plays to square A, and then to square B. In another phase, another piece of the same color plays to square B, and then to square A.
Suvorovskij Natisk, 1965
13th Russian Team Championship, 1999
British Chess Magazine, 1978