Function aggregate_formula 

fn aggregate_formula(
    formula_val: &RValue,
    positional: &[RValue],
    named: &[(String, RValue)],
    extra_named: &[(String, RValue)],
    fail_fast: bool,
    env: &Environment,
    context: &BuiltinContext<'_>,
) -> Result<RValue, RError>

Formula interface for aggregate: aggregate(y ~ x, data=df, FUN=mean)