diff --git a/src/context.rs b/src/context.rs
index fb49b1879..b3377f567 100644
--- a/src/context.rs
+++ b/src/context.rs
@@ -14,6 +14,7 @@ use crate::{
 /// or end of the file.
 const MAX_PADDING: usize = 3;
 
+// TODO: use FxHashMap here.
 pub fn opposite_positions(mps: &[MatchedPos]) -> HashMap<LineNumber, HashSet<LineNumber>> {
     let mut res: HashMap<LineNumber, HashSet<LineNumber>> = HashMap::new();