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diff --git a/readme.md b/readme.md
index fddc81b..7c28d83 100644
--- a/readme.md
+++ b/readme.md
@@ -20,3 +20,9 @@ Note: this is highly experimental and unstable.
 ## Usage
 
 `go run ./cmd/gint < ./samples/fib`
+
+To debug visually the AST, you can set the `DOT` env to a command able
+to a display a graphviz dot input, such as `dot -Txlib` or `dotty -`
+(for old graphviz versions like mine):
+
+`DOT="dot -Txlib" go run ./cmd/gint < ./samples/fib`