diff --git a/.github/ISSUE_TEMPLATE/bug_report.yaml b/.github/ISSUE_TEMPLATE/bug_report.yaml
new file mode 100644
index 00000000..2d210cad
--- /dev/null
+++ b/.github/ISSUE_TEMPLATE/bug_report.yaml
@@ -0,0 +1,58 @@
+name: 🐛 Bug report
+description: Create a report to help us improve
+labels:
+  - bug
+body:
+  - type: textarea
+    attributes:
+      label: Describe the bug?
+    validations:
+      required: true
+
+  - type: textarea
+    attributes:
+      label: Reproducible code
+    validations:
+      required: false
+
+  - type: textarea
+    attributes:
+      label: Expected result
+    validations:
+      required: false
+
+  - type: textarea
+    attributes:
+      label: Actual result
+    validations:
+      required: false
+
+  - type: textarea
+    attributes:
+      label: Additional context
+    validations:
+      required: false
+
+  - type: input
+    attributes:
+      label: Erg version
+    validations:
+      required: true
+
+  - type: input
+    attributes:
+      label: Python version
+    validations:
+      required: false
+
+  - type: dropdown
+    attributes:
+      label: os
+      options:
+        - Windows 10
+        - Windows 11
+        - MacOS 12 (Monterey)
+        - MacOS 11 (Big Sur)
+        - Ubuntu
+        - Linux(other distro)
+        - Other (write in `Additional context`)
diff --git a/compiler/erg_common/Cargo.toml b/compiler/erg_common/Cargo.toml
index b8c3f415..305f0b34 100644
--- a/compiler/erg_common/Cargo.toml
+++ b/compiler/erg_common/Cargo.toml
@@ -1,13 +1,13 @@
 [package]
 name = "erg_common"
-version = "0.5.8"
 description = "A common components library of Erg"
-authors = ["erg-lang team <moderation.erglang@gmail.com>"]
-license = "MIT OR Apache-2.0"
-edition = "2021"
-repository = "https://github.com/erg-lang/erg/tree/main/src/erg_common"
-documentation = "https://docs.rs/erg_common"
-homepage = "https://erg-lang.github.io/"
+version.workspace = true
+authors.workspace = true
+license.workspace = true
+edition.workspace = true
+repository.workspace = true
+documentation.workspace = true
+homepage.workspace = true
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
diff --git a/compiler/erg_compiler/Cargo.toml b/compiler/erg_compiler/Cargo.toml
index 1658fca2..e2f5a6c3 100644
--- a/compiler/erg_compiler/Cargo.toml
+++ b/compiler/erg_compiler/Cargo.toml
@@ -12,10 +12,18 @@ homepage.workspace = true
 
 [features]
 # when "debug" feature is turned on, that of parser will also be turned on.
-debug = [ "erg_common/debug", "erg_parser/debug", "erg_type/debug" ]
-japanese = [ "erg_common/japanese", "erg_parser/japanese", "erg_type/japanese" ]
-simplified_chinese = [ "erg_common/simplified_chinese", "erg_parser/simplified_chinese", "erg_type/simplified_chinese" ]
-traditional_chinese = [ "erg_common/traditional_chinese", "erg_parser/traditional_chinese", "erg_type/traditional_chinese" ]
+debug = ["erg_common/debug", "erg_parser/debug", "erg_type/debug"]
+japanese = ["erg_common/japanese", "erg_parser/japanese", "erg_type/japanese"]
+simplified_chinese = [
+    "erg_common/simplified_chinese",
+    "erg_parser/simplified_chinese",
+    "erg_type/simplified_chinese",
+]
+traditional_chinese = [
+    "erg_common/traditional_chinese",
+    "erg_parser/traditional_chinese",
+    "erg_type/traditional_chinese",
+]
 
 [dependencies]
 erg_common = { version = "0.5.8", path = "../erg_common" }
diff --git a/compiler/erg_compiler/codegen.rs b/compiler/erg_compiler/codegen.rs
index 0929f794..9a578702 100644
--- a/compiler/erg_compiler/codegen.rs
+++ b/compiler/erg_compiler/codegen.rs
@@ -2040,7 +2040,7 @@ impl CodeGenerator {
         self.stack_dec();
         self.emit_pop_top();
         self.emit_global_import_items(
-            Identifier::public("prelude"),
+            Identifier::public("_erg_std_prelude"),
             vec![(
                 Identifier::public("in_operator"),
                 Some(Identifier::private("#in_operator")),
diff --git a/compiler/erg_compiler/context/compare.rs b/compiler/erg_compiler/context/compare.rs
index f695ecb3..a5aca769 100644
--- a/compiler/erg_compiler/context/compare.rs
+++ b/compiler/erg_compiler/context/compare.rs
@@ -850,6 +850,9 @@ impl Context {
 
     /// returns union of two types (A or B)
     pub(crate) fn union(&self, lhs: &Type, rhs: &Type) -> Type {
+        if lhs == rhs {
+            return lhs.clone();
+        }
         // `?T or ?U` will not be unified
         // `Set!(?T, 3) or Set(?T, 3)` wii be unified to Set(?T, 3)
         if !lhs.is_unbound_var() && !rhs.is_unbound_var() {
@@ -889,6 +892,9 @@ impl Context {
 
     /// returns intersection of two types (A and B)
     pub(crate) fn intersection(&self, lhs: &Type, rhs: &Type) -> Type {
+        if lhs == rhs {
+            return lhs.clone();
+        }
         // ?T and ?U will not be unified
         if !lhs.is_unbound_var() && !rhs.is_unbound_var() {
             match (self.supertype_of(lhs, rhs), self.subtype_of(lhs, rhs)) {
diff --git a/compiler/erg_compiler/context/initialize/mod.rs b/compiler/erg_compiler/context/initialize/mod.rs
index 7365fb37..1b631095 100644
--- a/compiler/erg_compiler/context/initialize/mod.rs
+++ b/compiler/erg_compiler/context/initialize/mod.rs
@@ -1373,6 +1373,10 @@ impl Context {
         let mut record_type = Self::builtin_mono_class("RecordType", 2);
         record_type.register_superclass(builtin_mono("Record"), &record);
         record_type.register_superclass(builtin_mono("Type"), &type_);
+        /* Or (true or type) */
+        let or_t = builtin_poly("Or", vec![ty_tp(mono_q("L")), ty_tp(mono_q("R"))]);
+        let mut or = Self::builtin_poly_class("Or", vec![PS::t_nd("L"), PS::t_nd("R")], 2);
+        or.register_superclass(Obj, &obj);
         /* Float_mut */
         let mut float_mut = Self::builtin_mono_class("Float!", 2);
         float_mut.register_superclass(Float, &float);
@@ -1744,6 +1748,7 @@ impl Context {
         );
         self.register_builtin_type(builtin_mono("Record"), record, Const);
         self.register_builtin_type(builtin_mono("RecordType"), record_type, Const);
+        self.register_builtin_type(or_t, or, Const);
         self.register_builtin_type(builtin_mono("Int!"), int_mut, Const);
         self.register_builtin_type(builtin_mono("Nat!"), nat_mut, Const);
         self.register_builtin_type(builtin_mono("Float!"), float_mut, Const);
diff --git a/compiler/erg_compiler/context/inquire.rs b/compiler/erg_compiler/context/inquire.rs
index b138f31c..46f9bd7e 100644
--- a/compiler/erg_compiler/context/inquire.rs
+++ b/compiler/erg_compiler/context/inquire.rs
@@ -17,7 +17,7 @@ use erg_parser::ast::{self, Identifier};
 use erg_parser::token::Token;
 
 use erg_type::constructors::{
-    anon, builtin_mono, free_var, func, module, mono_proj, subr_t, v_enum,
+    anon, builtin_mono, builtin_poly, free_var, func, module, mono_proj, subr_t, v_enum,
 };
 use erg_type::free::Constraint;
 use erg_type::typaram::TyParam;
@@ -1454,7 +1454,7 @@ impl Context {
             }
             Type::Poly { path, name, .. } => {
                 if self.path() == path {
-                    if let Some((_, ctx)) = self.rec_get_mono_type(name) {
+                    if let Some((_, ctx)) = self.rec_get_poly_type(name) {
                         return Some(ctx);
                     }
                 }
@@ -1469,7 +1469,7 @@ impl Context {
                             .and_then(|cache| cache.ref_ctx(path.as_path()))
                     })
                 {
-                    if let Some((_, ctx)) = ctx.rec_get_mono_type(name) {
+                    if let Some((_, ctx)) = ctx.rec_get_poly_type(name) {
                         return Some(ctx);
                     }
                 }
@@ -1516,16 +1516,10 @@ impl Context {
                     return Some(res);
                 }
             }
-            Type::Or(l, r) => {
-                let lctx = self.get_nominal_type_ctx(l)?;
-                let rctx = self.get_nominal_type_ctx(r)?;
-                // use smaller context
-                return match (self.supertype_of(l, r), self.supertype_of(r, l)) {
-                    (true, true) => Some(lctx),
-                    (true, false) => Some(rctx),
-                    (false, true) => Some(lctx),
-                    (false, false) => None,
-                };
+            Type::Or(_l, _r) => {
+                if let Some(ctx) = self.get_nominal_type_ctx(&builtin_poly("Or", vec![])) {
+                    return Some(ctx);
+                }
             }
             // FIXME: `F()`などの場合、実際は引数が省略されていてもmonomorphicになる
             other if other.is_monomorphic() => {
diff --git a/compiler/erg_compiler/std/prelude.py b/compiler/erg_compiler/std/_erg_std_prelude.py
similarity index 100%
rename from compiler/erg_compiler/std/prelude.py
rename to compiler/erg_compiler/std/_erg_std_prelude.py
diff --git a/compiler/erg_compiler/std/prelude.er b/compiler/erg_compiler/std/prelude.er
index b0c0df8f..5f01c9ea 100644
--- a/compiler/erg_compiler/std/prelude.er
+++ b/compiler/erg_compiler/std/prelude.er
@@ -2,7 +2,8 @@ discard _x = None
 
 discard 1
 
-cond c, then, else =
+# if: |T, U|(Bool, T, U) -> T or U
+cond|T: Type|(c: Bool, then: T, else: T): T =
     if c:
         do then
         do else
diff --git a/compiler/erg_parser/Cargo.toml b/compiler/erg_parser/Cargo.toml
index 1d33bb67..38288345 100644
--- a/compiler/erg_parser/Cargo.toml
+++ b/compiler/erg_parser/Cargo.toml
@@ -1,19 +1,19 @@
 [package]
 name = "erg_parser"
-version = "0.5.8"
 description = "The Erg parser"
-authors = ["erg-lang team <moderation.erglang@gmail.com>"]
-license = "MIT OR Apache-2.0"
-edition = "2021"
-repository = "https://github.com/erg-lang/erg/tree/main/src/erg_compiler/erg_parser"
-documentation = "https://docs.rs/erg_parser"
-homepage = "https://erg-lang.github.io/"
+version.workspace = true
+authors.workspace = true
+license.workspace = true
+edition.workspace = true
+repository.workspace = true
+documentation.workspace = true
+homepage.workspace = true
 
 [features]
-debug = [ "erg_common/debug" ]
-japanese = [ "erg_common/japanese" ]
-simplified_chinese = [ "erg_common/simplified_chinese" ]
-traditional_chinese = [ "erg_common/traditional_chinese" ]
+debug = ["erg_common/debug"]
+japanese = ["erg_common/japanese"]
+simplified_chinese = ["erg_common/simplified_chinese"]
+traditional_chinese = ["erg_common/traditional_chinese"]
 
 [dependencies]
 erg_common = { version = "0.5.8", path = "../erg_common" }
diff --git a/compiler/erg_parser/parse.rs b/compiler/erg_parser/parse.rs
index 739499ac..78d70a9c 100644
--- a/compiler/erg_parser/parse.rs
+++ b/compiler/erg_parser/parse.rs
@@ -469,7 +469,8 @@ impl Parser {
                         }
                     }
                 }
-                Some(t) if t.is(LSqBr) => {
+                // x[...] (`x [...]` will interpreted as `x([...])`)
+                Some(t) if t.is(LSqBr) && acc.col_end().unwrap() == t.col_begin().unwrap() => {
                     self.skip();
                     let index = self
                         .try_reduce_expr(false, false)
diff --git a/compiler/erg_type/Cargo.toml b/compiler/erg_type/Cargo.toml
index 39234fea..3727d03c 100644
--- a/compiler/erg_type/Cargo.toml
+++ b/compiler/erg_type/Cargo.toml
@@ -1,21 +1,21 @@
 [package]
 name = "erg_type"
-version = "0.5.8"
 description = "APIs for Erg types"
-authors = ["erg-lang team <moderation.erglang@gmail.com>"]
-license = "MIT OR Apache-2.0"
-edition = "2021"
-repository = "https://github.com/erg-lang/erg/tree/main/compiler/erg_type"
-documentation = "https://docs.rs/erg_type"
-homepage = "https://erg-lang.github.io/"
+version.workspace = true
+authors.workspace = true
+license.workspace = true
+edition.workspace = true
+repository.workspace = true
+documentation.workspace = true
+homepage.workspace = true
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [features]
-debug = [ "erg_common/debug" ]
-japanese = [ "erg_common/japanese" ]
-simplified_chinese = [ "erg_common/simplified_chinese" ]
-traditional_chinese = [ "erg_common/traditional_chinese" ]
+debug = ["erg_common/debug"]
+japanese = ["erg_common/japanese"]
+simplified_chinese = ["erg_common/simplified_chinese"]
+traditional_chinese = ["erg_common/traditional_chinese"]
 
 [dependencies]
 erg_common = { version = "0.5.8", path = "../erg_common" }
diff --git a/compiler/erg_type/lib.rs b/compiler/erg_type/lib.rs
index 1aa7a7d8..ea9d161c 100644
--- a/compiler/erg_type/lib.rs
+++ b/compiler/erg_type/lib.rs
@@ -1922,11 +1922,6 @@ impl Type {
         matches!(self, Self::FreeVar(_))
     }
 
-    /// FIXME: `Int or Str` should be monomorphic
-    pub fn is_monomorphic(&self) -> bool {
-        matches!(self.typarams_len(), Some(0) | None)
-    }
-
     pub const fn is_callable(&self) -> bool {
         matches!(self, Self::Subr { .. } | Self::Callable { .. })
     }
@@ -1935,6 +1930,18 @@ impl Type {
         matches!(self, Self::FreeVar(fv) if fv.is_unbound() || fv.crack().is_unbound_var())
     }
 
+    /// See also: `is_monomorphized`
+    pub fn is_monomorphic(&self) -> bool {
+        matches!(self.typarams_len(), Some(0) | None)
+    }
+
+    /// `Set(Int, 3)` is not monomorphic but monomorphized
+    pub fn is_monomorphized(&self) -> bool {
+        matches!(self.typarams_len(), Some(0) | None)
+            || (self.has_no_qvar() && self.has_no_unbound_var())
+    }
+
+    /// if the type is polymorphic
     pub fn has_qvar(&self) -> bool {
         match self {
             Self::MonoQVar(_) | Self::PolyQVar { .. } => true,
@@ -1972,6 +1979,10 @@ impl Type {
         }
     }
 
+    pub fn has_no_qvar(&self) -> bool {
+        !self.has_qvar()
+    }
+
     pub fn is_cachable(&self) -> bool {
         match self {
             Self::FreeVar(_) => false,
diff --git a/tests/fizzbuzz.er b/tests/fizzbuzz.er
index afcf9b73..b725528e 100644
--- a/tests/fizzbuzz.er
+++ b/tests/fizzbuzz.er
@@ -1,6 +1,6 @@
 for! 1..<100, i =>
-    match (i % 3, i % 5):
-        (0, 0) => print! "FizzBuzz"
-        (0, _) => print! "Fizz"
-        (_, 0) => print! "Buzz"
-        (_, _) => print! i
+    match [i % 3, i % 5]:
+        [0, 0] => print! "FizzBuzz"
+        [0, _] => print! "Fizz"
+        [_, 0] => print! "Buzz"
+        [_, _] => print! i
diff --git a/tests/test.rs b/tests/test.rs
index 0766dc6c..90ca0349 100644
--- a/tests/test.rs
+++ b/tests/test.rs
@@ -56,6 +56,11 @@ fn exec_move_check() -> Result<(), ()> {
     expect_failure("examples/move_check.er")
 }
 
+#[test]
+fn exec_prelude() -> Result<(), ()> {
+    expect_success("compiler/erg_compiler/std/prelude.er")
+}
+
 #[test]
 fn exec_quantified() -> Result<(), ()> {
     expect_success("examples/quantified.er")