Allows extension packages to make use of the formula parsing of specials.

new_specials(..., .required_specials = NULL, .xreg_specials = NULL)

## Arguments

... A named set of functions which used to parse formula inputs The names of specials which must be provided (and if not, are included with no inputs). The names of specials which will be only used as inputs to other specials (most commonly xreg).