diff --git a/src/4.1-data_struct.md b/src/4.1-data_struct.md
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@@ -1,256 +0,0 @@
-# Chatper 4.1 Key Data Structures of Rust Compiler
-
-## Toy Example
-
-**Source Rust code**
-```rust
-fn main() {
-    let bob = Box::new(1); 
-}
-```
-
-**HIR**
-Obtain the HIR of the source code 
-```
-cargo rustc -- -Z unpretty=hir-tree
-```
-```
-Crate {
-    owners: [
-        ...
-        Owner(
-            OwnerInfo {
-                nodes: OwnerNodes {
-                    node: Some(
-                        ParentedNode {
-                            parent: 4294967040,
-                            node: Item(
-                                Item {
-                                    ident: main#0,
-                                    owner_id: DefId(0:3 ~ toy[5100]::main),
-                                    kind: Fn(
-                                        FnSig {
-                                            header: FnHeader {
-                                                unsafety: Normal,
-                                                constness: NotConst,
-                                                asyncness: NotAsync,
-                                                abi: Rust,
-                                            },
-                                            decl: FnDecl {
-                                                inputs: [],
-                                                output: DefaultReturn(
-                                                    src/main.rs:1:10: 1:10 (#0),
-                                                ),
-                                                c_variadic: false,
-                                                implicit_self: None,
-                                                lifetime_elision_allowed: false,
-                                            },
-                                            span: src/main.rs:1:1: 1:10 (#0),
-                                        },
-                                        Generics {
-                                            params: [],
-                                            predicates: [],
-                                            has_where_clause_predicates: false,
-                                            where_clause_span: src/main.rs:1:10: 1:10 (#0),
-                                            span: src/main.rs:1:8: 1:8 (#0),
-                                        },
-                                        BodyId {
-                                            hir_id: HirId(DefId(0:3 ~ toy[5100]::main).11),
-                                        },
-                                    ),
-                                    span: src/main.rs:1:1: 3:2 (#0),
-                                    vis_span: no-location (#0),
-                                },
-                            ),
-                        },
-                    ),
-
-                    ...
-                    bodies: {
-                        11: Body {
-                            params: [],
-                            value: Expr {
-                                hir_id: HirId(DefId(0:3 ~ toy[5100]::main).11),
-                                kind: Block(
-                                    Block {
-                                        stmts: [
-                                            Stmt {
-                                                hir_id: HirId(DefId(0:3 ~ toy[5100]::main).1),
-                                                kind: Local(
-                                                    Local {
-                                                        pat: Pat {
-                                                            hir_id: HirId(DefId(0:3 ~ toy[5100]::main).9),
-                                                            kind: Binding(
-                                                                BindingAnnotation(
-                                                                    No,
-                                                                    Not,
-                                                                ),
-                                                                HirId(DefId(0:3 ~ toy[5100]::main).9),
-                                                                bob#0,
-                                                                None,
-                                                            ),
-                                                            span: src/main.rs:2:9: 2:12 (#0),
-                                                            default_binding_modes: true,
-                                                        },
-                                                        ty: None,
-                                                        init: Some(
-                                                            Expr {
-                                                                hir_id: HirId(DefId(0:3 ~ toy[5100]::main).2),
-                                                                kind: Call(
-                                                                    Expr {
-                                                                        hir_id: HirId(DefId(0:3 ~ toy[5100]::main).3),
-                                                                        kind: Path(
-                                                                            TypeRelative(
-                                                                                Ty {
-                                                                                    hir_id: HirId(DefId(0:3 ~ toy[5100]::main).5),
-                                                                                    kind: Path(
-                                                                                        Resolved(
-                                                                                            None,
-                                                                                            Path {
-                                                                                                span: src/main.rs:2:15: 2:18 (#0),
-                                                                                                res: Def(
-                                                                                                    Struct,
-                                                                                                    DefId(5:276 ~ alloc[1281]::boxed::Box),
-                                                                                                ),
-                                                                                                segments: [
-                                                                                                    PathSegment {
-                                                                                                        ident: Box#0,
-                                                                                                        hir_id: HirId(DefId(0:3 ~ toy[5100]::main).4),
-                                                                                                        res: Def(
-                                                                                                            Struct,
-                                                                                                            DefId(5:276 ~ alloc[1281]::boxed::Box),
-                                                                                                        ),
-                                                                                                        args: None,
-                                                                                                        infer_args: true,
-                                                                                                    },
-                                                                                                ],
-                                                                                            },
-                                                                                        ),
-                                                                                    ),
-                                                                                    span: src/main.rs:2:15: 2:18 (#0),
-                                                                                },
-                                                                                PathSegment {
-                                                                                    ident: new#0,
-                                                                                    hir_id: HirId(DefId(0:3 ~ toy[5100]::main).6),
-                                                                                    res: Err,
-                                                                                    args: None,
-                                                                                    infer_args: true,
-                                                                                },
-                                                                            ),
-                                                                        ),
-                                                                        span: src/main.rs:2:15: 2:23 (#0),
-                                                                    },
-                                                                    [
-                                                                        Expr {
-                                                                            hir_id: HirId(DefId(0:3 ~ toy[5100]::main).7),
-                                                                            kind: Lit(
-                                                                                Spanned {
-                                                                                    node: Int(
-                                                                                        1,
-                                                                                        Unsuffixed,
-                                                                                    ),
-                                                                                    span: src/main.rs:2:24: 2:25 (#0),
-                                                                                },
-                                                                            ),
-                                                                            span: src/main.rs:2:24: 2:25 (#0),
-                                                                        },
-                                                                    ],
-                                                                ),
-                                                                span: src/main.rs:2:15: 2:26 (#0),
-                                                            },
-                                                        ),
-                                                        els: None,
-                                                        hir_id: HirId(DefId(0:3 ~ toy[5100]::main).8),
-                                                        span: src/main.rs:2:5: 2:27 (#0),
-                                                        source: Normal,
-                                                    },
-                                                ),
-                                                span: src/main.rs:2:5: 2:27 (#0),
-                                            },
-                                        ],
-                                        expr: None,
-                                        hir_id: HirId(DefId(0:3 ~ toy[5100]::main).10),
-                                        rules: DefaultBlock,
-                                        span: src/main.rs:1:11: 3:2 (#0),
-                                        targeted_by_break: false,
-                                    },
-                                    None,
-                                ),
-                                span: src/main.rs:1:11: 3:2 (#0),
-                            },
-                            generator_kind: None,
-                        },
-                    },
-                    opt_hash_including_bodies: Some(
-                        Fingerprint(
-                            17038548455900965345,
-                            6128119105717993704,
-                        ),
-                    ),
-                },
-                parenting: {},
-                attrs: AttributeMap {
-                    map: {},
-                    opt_hash: Some(
-                        Fingerprint(
-                            17025902295854411478,
-                            11375155654212205663,
-                        ),
-                    ),
-                },
-                trait_map: {
-                    3: [],
-                },
-            },
-        ),
-    ],
-    opt_hir_hash: Some(
-        Fingerprint(
-            13212131797653419263,
-            9623007000143043823,
-        ),
-    ),
-}
-
-```
-
-
-**MIR**
-Obtain the MIR of the source code 
-```
-cargo rustc -- -Zunpretty=mir
-```
-
-```
-fn main() -> () {
-    let mut _0: ();
-    let _1: std::boxed::Box<i32>;
-    scope 1 {
-        debug bob => _1;
-    }
-
-    bb0: {
-        _1 = Box::<i32>::new(const 1_i32) -> [return: bb1, unwind continue];
-    }
-
-    bb1: {
-        drop(_1) -> [return: bb2, unwind continue];
-    }
-
-    bb2: {
-        return;
-    }
-}
-```
-
-## Terminalogies
-
- - [**TyCtxt**](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/ty/struct.TyCtxt.html) is the central data structure of Rust compilers. We can obtain the hir or mir of a function based on the object.
-```rust
-let hir = tcx.hir();
-let mir = optimized_mir(def_id); // def_id is of type DefId
-```
- - [**DefId**](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_span/def_id/struct.DefId.html): 
- - [**Local**](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.Local.html):
- - [**LocalDecl**](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.LocalDecl.html):
-