Function filter_by 

pub fn filter_by<F>(
    filters: Vec<F>,
    zipcodes: Option<Vec<Zipcode>>,
) -> Result<Vec<Zipcode>>
where
    F: Fn(&Zipcode) -> bool,

Using a supplied list of filt-er-functions, return a filtered list of zipcodes.

By default, the supplied list of zipcodes is everything stored in the database. However, an optional list of override zipcodes can be supplied.