.invert.single          Invert multiple elements of a multiple element
                        object
.is.one.sided           Determine if an object is one- or two- sided.
as.character.formula    Converts a formula to character
formula.parts           Manipulate the component parts of formulas,
                        expressions, calls, name/symbols and list and
                        vectors of such objects.
get.vars                Get variable (names) from various R objects
invert                  invert
is.cat                  Work with variables as categorical or
                        continuous
op.type                 Get the operator type used in an call, formula,
                        expression, etc.
terms.call              terms
