diff --git a/exercises/etl/src/lib.rs b/exercises/etl/src/lib.rs
index 8b1378917..4b643b0a7 100644
--- a/exercises/etl/src/lib.rs
+++ b/exercises/etl/src/lib.rs
@@ -1 +1,5 @@
+use std::collections::BTreeMap;
 
+pub fn transform(h: &BTreeMap<i32, Vec<char>>) -> BTreeMap<char, i32> {
+    unimplemented!("How will you transform the tree {:?}?", h)
+}