Function lower_str_with_externals 

pub fn lower_str_with_externals(
    ctx: &mut Context<'_>,
    source: &str,
    externals: HashMap<String, ValueId>,
) -> Result<Symbols, String>

Like lower_str, but externals supplies values for {name} capture atoms — used by the qcode! macro to inject in-scope Rust values into the program.