Formula Helpers
fill_dots(var_names, .)A character vector of variable names representing a single type of interaction
var_names may include the wildcard variable "." in which case the argument . must be specified
so that all interactions matching the form of var_names are generated.
Specification of variables for use in the formula.
This function takes a character vector var_names of the form c(name1, name2, ".", name3, ".")
with any number of nameint variables and any number of wild card variables ".".
It returns a list of character vectors of the form c(name1, name2, wildcard1, name3, wildcard2)
where wildcard1 and wildcard2 are iterated over all possible character names given in the argument ..