depth filter is a numeric filter whose value is the depth of the current position. Any mainline position has variation depth of 0.
If X is a position other than the initial position, then:
- if X is the primary child of its parent, its variation depth equals the variation depth of its parent
- if X is a not the primary child of its parent, its variation depth is the variation depth of its parent plus 1.
Thus the two lines below match the same set of positions: