Draft: Add basic tests

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 6.1
+// swift-tools-version: 6.0
 // The swift-tools-version declares the minimum version of Swift required to build this package.
 
 import PackageDescription

Tests/DifferentiationTests/DifferentiationTests.swift

@@ -1,6 +1,28 @@
 import Testing
 @testable import Differentiation
 
-@Test func example() async throws {
-    // Write your test here and use APIs like `#expect(...)` to check expected conditions.
+@Suite("Basic Differentiation") struct BasicDifferentiationTests {
+    @differentiable(reverse)
+    private func xSquared(x: Double) -> Double {
+        return x * x
+    }
+
+    private struct Vector2: Differentiable {
+        var x: Double
+        var y: Double
+    }
+
+    private func selfDotProduct(of vector: Vector2) -> Double {
+        return (vector.x * vector.x) + (vector.y * vector.y)
+    }
+
+    @Test func simpleFunctionDerivative() {
+        #expect(gradient(at: 3, of: xSquared) == 6.0)
+    }
+
+    @Test func simpleStructDerivative() {
+        let gradientOfDotProduct = gradient(at: Vector2(x: 5.0, y: 5.0), of: selfDotProduct)
+        #expect(gradientOfDotProduct == Vector2.TangentVector(x: 10.0, y: 10.0))
+    }
+
 }