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A general description is here.
The ‘plot’ method for ‘"hclust"’ objects gets an optional argument ‘check’; When that is true, as per default, it checks more carefully for valid input.
‘merge(<dendrogram>, ..)’ now works correctly for two `independent' dendrograms (PR#15648), and still compatibly via ‘adjust = "auto"’ e.g. for two branches of an existing dendrogram.