I got confused with how use works, assuming it required an explicit natural number (e.g. 37). Tweaking the docs to make it more clear that use accepts more than just natural numbers.

Game/Levels/LessOrEqual/L01le_refl.lean

@@ -18,7 +18,13 @@ that `x = 37` will work, then `use 37` will make progress.
 
 Because `a ≤ b` is notation for \"there exists `c` such that `b = a + c`\",
 you can make progress on goals of the form `a ≤ b` by `use`ing the
-number which is morally `b - a`.
+number which is morally `b - a` (i.e. `use b - a`)
+
+Any of the following examples is possible assuming the type of the argument passed to the `use` function is accurate:
+
+- `use 37`
+- `use a`
+- `use a * a + 1`
 -/
 TacticDoc use