Add formula terms from the original formula with character representation.
h_add_terms(f, adds, drop_response = FALSE)A new formula with elements in drops removed.
(formula)
a formula to be updated.
(character)
representation of elements to be added.
(flag)
whether response should be dropped.
Elements in adds will be added from the formula, while the environment
of the formula is unchanged. If adds is NULL or character(0), the formula is
unchanged.