diff --git a/src/main.shy b/src/main.shy
index d77a4d49..d3ac41cc 100644
--- a/src/main.shy
+++ b/src/main.shy
@@ -1,4 +1,11 @@
 title "Contexts"
+refer `
+https://www.unicode.org/charts/PDF/
+https://www.unicode.org/charts/PDF/U0000.pdf
+https://www.unicode.org/charts/PDF/U2300.pdf
+https://www.unicode.org/charts/PDF/U25A0.pdf
+https://www.unicode.org/charts/PDF/U2600.pdf
+`
 chapter "实践"
 label `
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