The function invert_sign() is the same as applying \( \frac{1}{x} \) with
the underlying numeric values of ptvalues. Times values (\(\times\)) will
be converted to div (\(\div\)) and div to times.
The function abs_sign() finds the multiplicative absolute values where all
times and div values are converted to times values (by default). It can also
converts to all div values by specifying sign = "div".
Functions as_times() and as_div() are wrappers of abs_sign() where
sign = "times" is specified for as_times() and sign = "div" is
specified for as_div().
All functions will return a vector of class ptvalue even when providing
a numeric vector for x as it is the expected use. To see the underlying
numeric vector, you can use unclass() or as.double().