A set of alternative topics is sort of like an inverted if-then-else. You start by defining a normal TopicEntry (an AskTopic, or an AskTellTopic, or whatever) for the basic response. Then, you add a nested AltTopic located within the base topic; you can add another AltTopic nested within the base topic, and another after that, and so on. When we need to choose one of the topics, we'll choose the last one that indicates it's active. So, the order of appearance is essentially an override order: the first AltTopic overrides its parent TopicEntry, and each subsequent AltTopic overrides its previous AltTopic.
+ AskTellTopic @lighthouse "It's very tall.";
++ AltTopic "Not really..." isActive=(...);
++ AltTopic "Well, maybe..." isActive=(...);
++ AltTopic "One more thing..." isActive=(...);
In this example, the response we'll show for ASK ABOUT LIGHTHOUSE will always be the LAST entry of the group that's active. For example, if all of the responses are active except for the very last one, then we'll show the "Well, maybe" response, because it's the last active response. If the main AskTellTopic is active, but none of the AltTopics are active, we'll show the "It's very tall" main response, because it's the last element of the group that's active.
Note that an AltTopic takes its matching information from its parent, so you don't need to specify a matchObj or any other matching information in an AltTopic. You merely need to provide the response text and the isActive test.
AltTopic : TopicEntry