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A general description is here.
‘predict(<rank-deficient lm>, newdata=*)’ now obeys a new argument ‘rankdeficient’, currently with default ‘"simple"’ for back compatibility, but that may well change before release. This addresses PR#15072 by Russ Lenth and is based on his original proposal and discussions in PR#16158 also by David Firth and Elin Waring. Experimental; may be updated before release.