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: model.rs
    path of file: ./repos/starship/src/formatter
    
    the code of the file until where you have to start completion: use std::borrow::Cow;
use std::collections::BTreeSet;

/// Type that holds a number of variables of type `T`
pub trait VariableHolder<T> {
    fn get_variables(&self) -> BTreeSet<T>;
}

/// Type that holds a number of style variables of type `T`
pub trai
    
    fn get_style_variables(&self) -> BTreeSet<Cow<'a, str>> {
        match self {
            StyleElement::Variable(var) => {
                let mut variables = BTreeSet::new();
                variables.insert(var.clone());
                variables
            }
            _ => Default::default(),
        }
    }