test(tests/lean/run): add test for issue #1089

See #1089

tests/lean/run/1089.lean

@@ -0,0 +1,21 @@
+import init.string
+import init.bool
+import system.io
+
+inductive token
+| eof : token
+| plus : token
+| var : string -> token
+
+open token
+open option
+
+def to_token : string → option token
+| [] := none
+| (c :: cs) :=
+  let t : option token := match c with
+    | #"x" := some (var "x")
+    | #"y" := some (var "y")
+    | #"+" := some plus
+    | _ := none
+  end in t