pub fn is_valid(&self) -> bool {
        match *self {
            Isbn::_10(ref c) => c.is_valid(),
            Isbn::_13(ref c) => c.is_valid(),
        }
    }
/// Returns `true` if this is a valid ISBN code.
Returns `true` if this is a valid ISBN code.
Returns ` true ` if this is a valid ISBN code .
pub fn is_valid(&self) -> bool {
        match *self {
            Isbn::_10(ref c) => c.is_valid(),
            Isbn::_13(ref c) => c.is_valid(),
        }
    }
pub fn is_valid ( & self ) -> bool { match * self { Isbn :: _10 ( ref c ) => c . is_valid ( ) , Isbn :: _13 ( ref c ) => c . is_valid ( ) , } }
Returns `true` if this is a valid ISBN code.
Returns ` true ` if this is a valid ISBN code .
&self