Create evaluation environment for specials

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

.required_specials

The names of specials which must be provided (and if not, are included with no inputs).

.xreg_specials

The names of specials which will be only used as inputs to other specials (most commonly xreg).