.invert.single          Invert multiple elements of a multiple element
                        object
as.character.formula    Converts a formula to character
env                     Get the environment
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.one.sided            Determine if an object is one- or two-sided.
                        Test whether a object (typically formula, call
                        or expression) is one- (e.g.  '~x') or
                        two-sided (e.g. 'x~y').
op.type                 Get the operator type used in an call, formula,
                        expression, etc.
split_terms             Split object into terms
terms.call              terms
toggle.sign             toggle the sign of an expression
