Let DummyCoding() specify a subset (or superset!) of levels #308
Description
Stata lets you economically state models with a subset of possible dummies for a given categorical variable:
regress y i(1 3).x
That regresses y on two dummies, one for x==1, one for x==3.
It would be nice for the levels option of DummyCoding() to allow the same.
One can also do
regress y ibn.x, nocons
Which specifies a complete dummy set and no constant term. ("bn" means "base, none") DummyCoding() could accept base=:none to do the same.