diff --git a/TODO.md b/TODO.md
index 47284b0..b9e5b54 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,6 +1,4 @@
-rename lang0 to lang
+# docs
 
-# lang0
-
-[lang0] docs/lambda-encoding -- Exp Lambda 编码的例子，外加解释器
-[lang0] docs/self-type
+[docs] docs/lambda-encoding -- Exp Lambda 编码的例子，外加解释器
+[docs] docs/self-type
diff --git a/src/command-line/commands/RunCommand.ts b/src/command-line/commands/RunCommand.ts
index b534f08..6549230 100644
--- a/src/command-line/commands/RunCommand.ts
+++ b/src/command-line/commands/RunCommand.ts
@@ -2,7 +2,7 @@ import { Command, CommandRunner } from "@xieyuheng/command-line"
 import { ty } from "@xieyuheng/ty"
 import fs from "fs"
 import Path from "path"
-import { run } from "../../lang0/run/index.js"
+import { run } from "../../lang/run/index.js"
 
 type Args = { file: string }
 type Opts = {}
diff --git a/src/lang0/actions/doAp.ts b/src/lang/actions/doAp.ts
similarity index 100%
rename from src/lang0/actions/doAp.ts
rename to src/lang/actions/doAp.ts
diff --git a/src/lang0/actions/index.ts b/src/lang/actions/index.ts
similarity index 100%
rename from src/lang0/actions/index.ts
rename to src/lang/actions/index.ts
diff --git a/src/lang0/definition/Definition.ts b/src/lang/definition/Definition.ts
similarity index 100%
rename from src/lang0/definition/Definition.ts
rename to src/lang/definition/Definition.ts
diff --git a/src/lang0/definition/index.ts b/src/lang/definition/index.ts
similarity index 100%
rename from src/lang0/definition/index.ts
rename to src/lang/definition/index.ts
diff --git a/src/lang0/env/Env.ts b/src/lang/env/Env.ts
similarity index 100%
rename from src/lang0/env/Env.ts
rename to src/lang/env/Env.ts
diff --git a/src/lang0/env/index.ts b/src/lang/env/index.ts
similarity index 100%
rename from src/lang0/env/index.ts
rename to src/lang/env/index.ts
diff --git a/src/lang0/equivalent/EquivalentCtx.ts b/src/lang/equivalent/EquivalentCtx.ts
similarity index 100%
rename from src/lang0/equivalent/EquivalentCtx.ts
rename to src/lang/equivalent/EquivalentCtx.ts
diff --git a/src/lang0/equivalent/equivalent.ts b/src/lang/equivalent/equivalent.ts
similarity index 100%
rename from src/lang0/equivalent/equivalent.ts
rename to src/lang/equivalent/equivalent.ts
diff --git a/src/lang0/equivalent/equivalentNeutral.ts b/src/lang/equivalent/equivalentNeutral.ts
similarity index 100%
rename from src/lang0/equivalent/equivalentNeutral.ts
rename to src/lang/equivalent/equivalentNeutral.ts
diff --git a/src/lang0/equivalent/index.ts b/src/lang/equivalent/index.ts
similarity index 100%
rename from src/lang0/equivalent/index.ts
rename to src/lang/equivalent/index.ts
diff --git a/src/lang0/evaluate/evaluate.ts b/src/lang/evaluate/evaluate.ts
similarity index 100%
rename from src/lang0/evaluate/evaluate.ts
rename to src/lang/evaluate/evaluate.ts
diff --git a/src/lang0/evaluate/evaluateDefinition.ts b/src/lang/evaluate/evaluateDefinition.ts
similarity index 100%
rename from src/lang0/evaluate/evaluateDefinition.ts
rename to src/lang/evaluate/evaluateDefinition.ts
diff --git a/src/lang0/evaluate/index.ts b/src/lang/evaluate/index.ts
similarity index 100%
rename from src/lang0/evaluate/index.ts
rename to src/lang/evaluate/index.ts
diff --git a/src/lang0/exp/Exp.ts b/src/lang/exp/Exp.ts
similarity index 100%
rename from src/lang0/exp/Exp.ts
rename to src/lang/exp/Exp.ts
diff --git a/src/lang0/exp/expFreeNames.ts b/src/lang/exp/expFreeNames.ts
similarity index 100%
rename from src/lang0/exp/expFreeNames.ts
rename to src/lang/exp/expFreeNames.ts
diff --git a/src/lang0/exp/expIndirectFreeNames.ts b/src/lang/exp/expIndirectFreeNames.ts
similarity index 100%
rename from src/lang0/exp/expIndirectFreeNames.ts
rename to src/lang/exp/expIndirectFreeNames.ts
diff --git a/src/lang0/exp/index.ts b/src/lang/exp/index.ts
similarity index 100%
rename from src/lang0/exp/index.ts
rename to src/lang/exp/index.ts
diff --git a/src/lang0/format/formatExp.ts b/src/lang/format/formatExp.ts
similarity index 95%
rename from src/lang0/format/formatExp.ts
rename to src/lang/format/formatExp.ts
index ce593a8..4c3f211 100644
--- a/src/lang0/format/formatExp.ts
+++ b/src/lang/format/formatExp.ts
@@ -1,4 +1,4 @@
-import type { Binding } from "../../lang0/substitution/index.js"
+import type { Binding } from "../../lang/substitution/index.js"
 import { type Exp } from "../exp/index.js"
 import { substitutionBindings } from "../substitution/Substitution.js"
 
diff --git a/src/lang0/format/index.ts b/src/lang/format/index.ts
similarity index 100%
rename from src/lang0/format/index.ts
rename to src/lang/format/index.ts
diff --git a/src/lang0/mod/Mod.ts b/src/lang/mod/Mod.ts
similarity index 100%
rename from src/lang0/mod/Mod.ts
rename to src/lang/mod/Mod.ts
diff --git a/src/lang0/mod/createMod.ts b/src/lang/mod/createMod.ts
similarity index 100%
rename from src/lang0/mod/createMod.ts
rename to src/lang/mod/createMod.ts
diff --git a/src/lang0/mod/index.ts b/src/lang/mod/index.ts
similarity index 100%
rename from src/lang0/mod/index.ts
rename to src/lang/mod/index.ts
diff --git a/src/lang0/mod/modDefine.ts b/src/lang/mod/modDefine.ts
similarity index 100%
rename from src/lang0/mod/modDefine.ts
rename to src/lang/mod/modDefine.ts
diff --git a/src/lang0/mod/modFind.ts b/src/lang/mod/modFind.ts
similarity index 100%
rename from src/lang0/mod/modFind.ts
rename to src/lang/mod/modFind.ts
diff --git a/src/lang0/mod/modFindValue.ts b/src/lang/mod/modFindValue.ts
similarity index 100%
rename from src/lang0/mod/modFindValue.ts
rename to src/lang/mod/modFindValue.ts
diff --git a/src/lang0/mod/modOwnDefinitions.ts b/src/lang/mod/modOwnDefinitions.ts
similarity index 100%
rename from src/lang0/mod/modOwnDefinitions.ts
rename to src/lang/mod/modOwnDefinitions.ts
diff --git a/src/lang0/mod/modResolve.ts b/src/lang/mod/modResolve.ts
similarity index 100%
rename from src/lang0/mod/modResolve.ts
rename to src/lang/mod/modResolve.ts
diff --git a/src/lang0/neutral/Neutral.ts b/src/lang/neutral/Neutral.ts
similarity index 100%
rename from src/lang0/neutral/Neutral.ts
rename to src/lang/neutral/Neutral.ts
diff --git a/src/lang0/neutral/index.ts b/src/lang/neutral/index.ts
similarity index 100%
rename from src/lang0/neutral/index.ts
rename to src/lang/neutral/index.ts
diff --git a/src/lang0/readback/ReadbackCtx.ts b/src/lang/readback/ReadbackCtx.ts
similarity index 100%
rename from src/lang0/readback/ReadbackCtx.ts
rename to src/lang/readback/ReadbackCtx.ts
diff --git a/src/lang0/readback/index.ts b/src/lang/readback/index.ts
similarity index 100%
rename from src/lang0/readback/index.ts
rename to src/lang/readback/index.ts
diff --git a/src/lang0/readback/readback.ts b/src/lang/readback/readback.ts
similarity index 100%
rename from src/lang0/readback/readback.ts
rename to src/lang/readback/readback.ts
diff --git a/src/lang0/readback/readbackNeutral.ts b/src/lang/readback/readbackNeutral.ts
similarity index 100%
rename from src/lang0/readback/readbackNeutral.ts
rename to src/lang/readback/readbackNeutral.ts
diff --git a/src/lang0/run/assertAllNamesDefined.ts b/src/lang/run/assertAllNamesDefined.ts
similarity index 100%
rename from src/lang0/run/assertAllNamesDefined.ts
rename to src/lang/run/assertAllNamesDefined.ts
diff --git a/src/lang0/run/assertEqual.ts b/src/lang/run/assertEqual.ts
similarity index 100%
rename from src/lang0/run/assertEqual.ts
rename to src/lang/run/assertEqual.ts
diff --git a/src/lang0/run/assertNotEqual.ts b/src/lang/run/assertNotEqual.ts
similarity index 100%
rename from src/lang0/run/assertNotEqual.ts
rename to src/lang/run/assertNotEqual.ts
diff --git a/src/lang0/run/define.ts b/src/lang/run/define.ts
similarity index 100%
rename from src/lang0/run/define.ts
rename to src/lang/run/define.ts
diff --git a/src/lang0/run/execute.ts b/src/lang/run/execute.ts
similarity index 100%
rename from src/lang0/run/execute.ts
rename to src/lang/run/execute.ts
diff --git a/src/lang0/run/importOne.ts b/src/lang/run/importOne.ts
similarity index 100%
rename from src/lang0/run/importOne.ts
rename to src/lang/run/importOne.ts
diff --git a/src/lang0/run/index.ts b/src/lang/run/index.ts
similarity index 100%
rename from src/lang0/run/index.ts
rename to src/lang/run/index.ts
diff --git a/src/lang0/run/load.ts b/src/lang/run/load.ts
similarity index 100%
rename from src/lang0/run/load.ts
rename to src/lang/run/load.ts
diff --git a/src/lang0/run/occurCheck.ts b/src/lang/run/occurCheck.ts
similarity index 100%
rename from src/lang0/run/occurCheck.ts
rename to src/lang/run/occurCheck.ts
diff --git a/src/lang0/run/run.ts b/src/lang/run/run.ts
similarity index 100%
rename from src/lang0/run/run.ts
rename to src/lang/run/run.ts
diff --git a/src/lang0/run/runMod.ts b/src/lang/run/runMod.ts
similarity index 100%
rename from src/lang0/run/runMod.ts
rename to src/lang/run/runMod.ts
diff --git a/src/lang0/stmt/Stmt.ts b/src/lang/stmt/Stmt.ts
similarity index 100%
rename from src/lang0/stmt/Stmt.ts
rename to src/lang/stmt/Stmt.ts
diff --git a/src/lang0/stmt/index.ts b/src/lang/stmt/index.ts
similarity index 100%
rename from src/lang0/stmt/index.ts
rename to src/lang/stmt/index.ts
diff --git a/src/lang0/substitution/Substitution.ts b/src/lang/substitution/Substitution.ts
similarity index 100%
rename from src/lang0/substitution/Substitution.ts
rename to src/lang/substitution/Substitution.ts
diff --git a/src/lang0/substitution/index.ts b/src/lang/substitution/index.ts
similarity index 100%
rename from src/lang0/substitution/index.ts
rename to src/lang/substitution/index.ts
diff --git a/src/lang0/syntax/Parser.ts b/src/lang/syntax/Parser.ts
similarity index 100%
rename from src/lang0/syntax/Parser.ts
rename to src/lang/syntax/Parser.ts
diff --git a/src/lang0/syntax/index.ts b/src/lang/syntax/index.ts
similarity index 100%
rename from src/lang0/syntax/index.ts
rename to src/lang/syntax/index.ts
diff --git a/src/lang0/syntax/matchExp.ts b/src/lang/syntax/matchExp.ts
similarity index 100%
rename from src/lang0/syntax/matchExp.ts
rename to src/lang/syntax/matchExp.ts
diff --git a/src/lang0/syntax/matchStmt.ts b/src/lang/syntax/matchStmt.ts
similarity index 100%
rename from src/lang0/syntax/matchStmt.ts
rename to src/lang/syntax/matchStmt.ts
diff --git a/src/lang0/utils/freshen.test.ts b/src/lang/utils/freshen.test.ts
similarity index 100%
rename from src/lang0/utils/freshen.test.ts
rename to src/lang/utils/freshen.test.ts
diff --git a/src/lang0/utils/freshen.ts b/src/lang/utils/freshen.ts
similarity index 100%
rename from src/lang0/utils/freshen.ts
rename to src/lang/utils/freshen.ts
diff --git a/src/lang0/value/Value.ts b/src/lang/value/Value.ts
similarity index 100%
rename from src/lang0/value/Value.ts
rename to src/lang/value/Value.ts
diff --git a/src/lang0/value/index.ts b/src/lang/value/index.ts
similarity index 100%
rename from src/lang0/value/index.ts
rename to src/lang/value/index.ts
diff --git a/src/lang0/value/lazyActive.ts b/src/lang/value/lazyActive.ts
similarity index 100%
rename from src/lang0/value/lazyActive.ts
rename to src/lang/value/lazyActive.ts