[x] if subset argument is the default, then internally subset the rows of data to rownames(model.matrix(first_mod)).
[x] throw a message that this happened. EDIT: Didn't do this. documented in man instead
[x] document in help page, and note how this is not guaranteed to deal with either 1) different underlying data frames in first_mod vs. main. EDIT: This was already done.
subset
argument is the default, then internally subset the rows ofdata
torownames(model.matrix(first_mod))
.