The levels of a factor are re-ordered so that the level specified by
ref is first and the others are moved down. This is useful
for contr.treatment contrasts which take the first level as
relevel(x, ref, …)
an unordered factor.
the reference level, typically a string.
additional arguments for future methods.
A factor of the same length as x.
This, as reorder(), is a special case of simply calling
factor(x, levels = levels(x)[....]).