gl(n, k, units=units(), labels=1:n, ordered=false) units(nunits)
gl generates factors by specifying the
pattern of their levels.
The result has levels from 1 to n
with each value replicated in groups of length
k out to a total length of units.
Labels for the resulting factor levels can be
optionally specified with the arguments labels
and the factor levels can be made ordered by
specifying ordered=true.
The function units can be used to declare a
default value for the length of the resulting factor.
gl is modelled on the Glim function of the
same name.