Function merge 

pub fn merge(lead: &str, other: &str, region: Option<&Region>) -> String

Two cards made one. lead stays as it is: its name, and its organisation, birthday and photo when it has them. other adds what only it has: numbers and e-mail addresses (the same ones written twice then cleaned, clean), postal addresses, web sites and chat addresses unless the same, its categories, its notes after a blank line (unless one holds the other), and what lead lacks of one value. Apple’s groups get names of their own; a vCard 4.0 line going into a 3.0 card is written as 3.0. REV renewed.