Depending on the merge mode, overlays and extensions are either merged with the master, or their trees are joined via aggregation
Forcefully sets a master unit for this API, which may be different from the one, current unit points to via masterRef.
Used for test comparison of two trees. Touching this will require AST tests update.
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Externally set master AST, should be only available for root nodes, and only in the case when we merge multiple overlays/extensions.