diff --git a/src/internal_type_traits.rs b/src/internal_type_traits.rs
index 8b13789..b4f060f 100644
--- a/src/internal_type_traits.rs
+++ b/src/internal_type_traits.rs
@@ -1 +1,90 @@
+use std::{
+    fmt,
+    iter::{Product, Sum},
+    ops::{
+        Add, AddAssign, BitAnd, BitAndAssign, BitOr, BitOrAssign, BitXor, BitXorAssign, Div,
+        DivAssign, Mul, MulAssign, Not, Rem, RemAssign, Shl, ShlAssign, Shr, ShrAssign, Sub,
+        SubAssign,
+    },
+};
 
+// Skipped:
+//
+// - `is_signed_int_t<T>`   (probably won't be used directly in `modint.rs`)
+// - `is_unsigned_int_t<T>` (probably won't be used directly in `modint.rs`)
+// - `to_unsigned_t<T>`     (not used in `fenwicktree.rs`)
+
+/// Corresponds to `std::is_integral` in C++.
+// We will remove unnecessary bounds later.
+//
+// Maybe we should rename this to `PrimitiveInteger` or something, as it probably won't be used in the
+// same way as the original ACL.
+pub(crate) trait Integral:
+    'static
+    + Send
+    + Sync
+    + Copy
+    + Ord
+    + Not<Output = Self>
+    + Add<Output = Self>
+    + Sub<Output = Self>
+    + Mul<Output = Self>
+    + Div<Output = Self>
+    + Rem<Output = Self>
+    + AddAssign
+    + SubAssign
+    + MulAssign
+    + DivAssign
+    + RemAssign
+    + Sum
+    + Product
+    + BitOr<Output = Self>
+    + BitAnd<Output = Self>
+    + BitXor<Output = Self>
+    + BitOrAssign
+    + BitAndAssign
+    + BitXorAssign
+    + Shl<Output = Self>
+    + Shr<Output = Self>
+    + ShlAssign
+    + ShrAssign
+    + fmt::Display
+    + fmt::Debug
+    + fmt::Binary
+    + fmt::Octal
+{
+    fn zero() -> Self;
+    fn one() -> Self;
+    fn min_value() -> Self;
+    fn max_value() -> Self;
+}
+
+macro_rules! impl_integral {
+    ($($ty:ty),*) => {
+        $(
+            impl Integral for $ty {
+                #[inline]
+                fn zero() -> Self {
+                    0
+                }
+
+                #[inline]
+                fn one() -> Self {
+                    1
+                }
+
+                #[inline]
+                fn min_value() -> Self {
+                    Self::min_value()
+                }
+
+                #[inline]
+                fn max_value() -> Self {
+                    Self::max_value()
+                }
+            }
+        )*
+    };
+}
+
+impl_integral!(i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize);