expandIfNecessary

open override fun <AdaptorMemento> expandIfNecessary(    signatureExtrasExtractor: (Element) -> Pair<A_Type?, List<A_Type>>,     adaptor: LookupTreeAdaptor<Element, Result, AdaptorMemento>,     numNaturalArgs: Int,     memento: AdaptorMemento): DecisionStep<Element, Result>

If it has not already been computed, compute and cache the decisionStep to use for making progress at this InternalLookupTree.