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: meta_var.rs
    path of file: ./repos/ast-grep/crates/core/src
    
    the code of the file until where you have to start completion: use crate::match_tree::does_node_match_exactly;
use crate::matcher::Matcher;
use crate::source::Content;
use crate::{Doc, Language, Node, StrDoc};
use std::borrow::Cow;
use std::collections::HashMap;

pub type MetaVariableID = String;

type Underlying<D> = Vec<<<D as Doc>::Source as Content>::Underlying>;
/// a dictionary that stores metavariable instantiation
/// const a = 123 matched with const a = $A will produce env: $A => 123
#[derive(Clone)]
pub struct MetaVarEnv<'tree, D: Doc> {
  single_matched: HashM
    
  fn match_multi_var(&self, id: &str, cands: &[Node<D>]) -> bool {
    let Some(nodes) = self.multi_matched.get(id) else {
      return true;
    };
    let mut named_nodes = nodes.iter().filter(|n| n.is_named());
    let mut named_cands = cands.iter().filter(|n| n.is_named());
    loop {
      if let Some(node) = named_nodes.next() {
        let Some(cand) = named_cands.next() else {
          // cand is done but node is not
          break false;
        };
        if !does_node_match_exactly(node, cand) {
          break false;
        }
      } else if named_cands.next().is_some() {
        // node is done but cand is not
        break false;
      } else {
        // both None, matches
        break true;
      }
    }
  }