%+%-methods             Utility functions to treat characters
correlation             Bivariate correlations
cross                   Crosstabs
freq                    Frequencies
print.cross             Crosstabs print methods
sum.n                   sum weighted on the number of non-missing
                        values
xf                      Methods that apply a function across a levels
                        of one or more factors
xtab                    Crosstabulations using formula
