expand
abstract fun <AdaptorMemento> expandIfNecessary( signatureExtrasExtractor: (Element) -> Pair<A_Type?, List<A_Type>>, adaptor: LookupTreeAdaptor<Element, Result, AdaptorMemento>, numNaturalArgs: Int, memento: AdaptorMemento): DecisionStep<Element, Result>
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If it has not already been computed, compute and cache the DecisionStep to use for making progress at this InternalLookupTree. Fail if this is not an InternalLookupTree.