You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: validation.rs
    path of file: ./repos/holochain-rust/crates/core_types/src
    
    the code of the file until where you have to start completion: //! This module defines structs that are used in the interchange
//! of data that is used for validation of chain modi
    
    pub fn sources(&self) -> Vec<Address> {
        self.package
            .chain_header
            .provenances()
            .iter()
            .map(|provenance| provenance.source())
            .collect()
    }