lazy
Answer the bundle trees that are waiting for a specific case-insensitive token to be parsed. These are organized as a map where each key is the lower-case string form of an expected case-insensitive token, and the corresponding value is the successor bundle tree representing the situation where a token case-insensitively matching the key was consumed.
This is only an authoritative map if an expand has been invoked since the last modification via methods like addPlanInProgress.
Return
A map from lowercase strings to bundle trees.