In Opal, the hierarchy among topics is described metaphorically in terms of generational relationships among females.
Thus, one topic (B) is the daughter of another topic (A) if B is a subtopic of A. (And in that case, A is B’s mother.)
Every topic has a mother, unless it is at top level. But a topic need not have any daughters.
A topic’s daughters are sisters of one another. Sisters are ordered among themselves, so a topic with daughters has a first daughter and a last daughter.
A topic with daughters may show or hide them (the topic is expanded or collapsed).