in (i) the domain of X (a head) is the set of nodes UP, ZP, WP, YP, and H and whatever these categories dominate. XP1, XP2, X', X1 and X2 are not included in the domain of X, since each of these nodes contain X, and X is not distinct from X.
XP1 /\ / \ UP XP2 /\ / \ ZP1 X' /\ /\ / \ / \ WP ZP2 X1 YP /\ / \ H X2
- Chomsky, N. 1993. A Minimalist Program for Linguistic Theory, MIT occasional papers in linguistics, 1-67. Reprinted in: Chomsky (1995).