diff --git a/.travis.yml b/.travis.yml
index 92406172..86399718 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -1,11 +1,16 @@
 matrix:
   include:
+    - os: osx
+      osx_image: xcode9.2
+      env: SWIFT_VERSION=4.0.3
     - os: osx
       osx_image: xcode9.4
       env: SWIFT_VERSION=4.1
     - os: osx
       osx_image: xcode10
       env: SWIFT_VERSION=4.2
+    - os: linux
+      env: SWIFT_VERSION=4.0.3
     - os: linux
       env: SWIFT_VERSION=4.1
     - os: linux
diff --git a/Package.swift b/Package.swift
index 2dd69776..b5fb3049 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,4 +1,4 @@
-// swift-tools-version:4.1
+// swift-tools-version:4.0
 import PackageDescription
 
 let package = Package(
diff --git a/Sources/_SwiftSupport.swift b/Sources/_SwiftSupport.swift
new file mode 100644
index 00000000..53336597
--- /dev/null
+++ b/Sources/_SwiftSupport.swift
@@ -0,0 +1,38 @@
+import Foundation
+
+#if !swift(>=4.1)
+  public extension Sequence {
+    func compactMap<ElementOfResult>(_ transform: (Element) throws -> ElementOfResult?) rethrows -> [ElementOfResult] {
+      return try flatMap(transform)
+    }
+  }
+#endif
+
+#if !swift(>=4.1)
+  public extension Collection {
+    func index(_ i: Self.Index, offsetBy n: Int) -> Self.Index {
+        let indexDistance = Self.IndexDistance(n)
+        return index(i, offsetBy: indexDistance)
+    }
+  }
+#endif
+
+#if !swift(>=4.1)
+public extension TemplateSyntaxError {
+  public static func ==(lhs: TemplateSyntaxError, rhs: TemplateSyntaxError) -> Bool {
+    return lhs.reason == rhs.reason &&
+      lhs.description == rhs.description &&
+      lhs.token == rhs.token &&
+      lhs.stackTrace == rhs.stackTrace &&
+      lhs.templateName == rhs.templateName
+  }
+}
+#endif
+
+#if !swift(>=4.1)
+public extension Variable {
+  public static func ==(lhs: Variable, rhs: Variable) -> Bool {
+    return lhs.variable == rhs.variable
+  }
+}
+#endif