Converts a no-intercept formula to the corresponding formula
with an intercept while preserving the formula environment. Additive,
parenthesized, and unary-plus no-intercept encodings such as - 1,
+ 0, and 0 + are removed without editing transformed calls
such as I(x - 1) or offset(x - 1).
formula_add_intercept(formula)A formula object with an intercept.