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+# file name change
+
+change module system and `use` clauses.
+
+This means we would not change all possiable places for now.(I think this is hard to implement even in future.)
+
+## module system
+
+### physical module and logic module
+
+each file and folder(lib.rs in the folder) is a physical module.
+
+`use` and `pub` could import and export module as logic.
+
+logic module could be seen as `namespace`.
+
+Only logic module matters.
+
+### module tree
+
+### module declaration
+
+> logic module could be defined in its ancestor's folder.
+
+> module
+
+## use clauses
+