diff --git a/.swift-version b/.swift-version
index 8c50098..5186d07 100644
--- a/.swift-version
+++ b/.swift-version
@@ -1 +1 @@
-3.1
+4.0
diff --git a/Package.swift b/Package.swift
index 1320542..23f76c5 100644
--- a/Package.swift
+++ b/Package.swift
@@ -1,5 +1,25 @@
+// swift-tools-version:4.0
 import PackageDescription
 
 let package = Package(
-    name: "SwiftShell"
+    name: "SwiftShell",
+    products: [
+        .library(
+            name: "SwiftShell",
+            targets: ["SwiftShell"])
+    ],
+    targets: [
+        .target(
+            name: "SwiftShell"),
+        .testTarget(
+            name: "SwiftShellTests",
+            dependencies: ["SwiftShell"]),
+        .testTarget(
+            name: "StreamTests",
+            dependencies: ["SwiftShell"]),
+        .testTarget(
+            name: "GeneralTests",
+            dependencies: ["SwiftShell"]),
+    ],
+    swiftLanguageVersions: [4]
 )
diff --git a/Sources/SwiftShell/Command.swift b/Sources/SwiftShell/Command.swift
index b2811b5..f4ff052 100755
--- a/Sources/SwiftShell/Command.swift
+++ b/Sources/SwiftShell/Command.swift
@@ -128,8 +128,6 @@ public func == (e1: CommandError, e2: CommandError) -> Bool {
 		return c1 == c2
 	case (.inAccessibleExecutable, .returnedErrorCode), (.returnedErrorCode, .inAccessibleExecutable):
 		return false
-	default:
-		fatalError("not implemented")
 	}
 }
 
diff --git a/Sources/SwiftShell/Stream/Lazy-split.swift b/Sources/SwiftShell/Stream/Lazy-split.swift
index 055d8ea..9f9a9b7 100644
--- a/Sources/SwiftShell/Stream/Lazy-split.swift
+++ b/Sources/SwiftShell/Stream/Lazy-split.swift
@@ -19,12 +19,9 @@ extension Collection where Iterator.Element: Equatable {
 /// A sequence from splitting a Collection lazily.
 public struct LazySplitSequence <Base: Collection>: IteratorProtocol, LazySequenceProtocol where
 	Base.Iterator.Element: Equatable,
-	Base.SubSequence: Collection,
-	Base.SubSequence.Iterator.Element==Base.Iterator.Element,
-	Base.SubSequence==Base.SubSequence.SubSequence {
-
+	Base.SubSequence: Collection {
 	public fileprivate(set) var remaining: Base.SubSequence?
-	public let separator: Base.Generator.Element
+	public let separator: Base.Iterator.Element
 	public let allowEmptySlices: Bool
 
 	/// Creates a lazy sequence by splitting a Collection repeatedly.
@@ -50,9 +47,7 @@ public struct LazySplitSequence <Base: Collection>: IteratorProtocol, LazySequen
 }
 
 extension LazyCollectionProtocol where Elements.Iterator.Element: Equatable,
-	Elements.SubSequence: Collection,
-	Elements.SubSequence.Iterator.Element==Elements.Iterator.Element,
-	Elements.SubSequence==Elements.SubSequence.SubSequence {
+	Elements.SubSequence: Collection {
 
 	/// Creates a lazy sequence by splitting this Collection repeatedly.
 	///
@@ -71,9 +66,7 @@ extension LazyCollectionProtocol where Elements.Iterator.Element: Equatable,
 /// A sequence from splitting a series of Collections lazily, as if they were one Collection.
 public struct PartialSourceLazySplitSequence <Base: Collection>: IteratorProtocol, LazySequenceProtocol where
 	Base.Iterator.Element: Equatable,
-	Base.SubSequence: RangeReplaceableCollection,
-	Base.SubSequence.Iterator.Element==Base.Iterator.Element,
-	Base.SubSequence==Base.SubSequence.SubSequence {
+	Base.SubSequence: RangeReplaceableCollection {
 
 	private var gs: LazyMapIterator<AnyIterator<Base>, LazySplitSequence<Base>>
 	private var g: LazySplitSequence<Base>?
diff --git a/SwiftShell.xcodeproj/xcshareddata/xcschemes/SwiftShell-iOS.xcscheme b/SwiftShell.xcodeproj/xcshareddata/xcschemes/SwiftShell-iOS.xcscheme
index ba25c0e..c9fe9a7 100644
--- a/SwiftShell.xcodeproj/xcshareddata/xcschemes/SwiftShell-iOS.xcscheme
+++ b/SwiftShell.xcodeproj/xcshareddata/xcschemes/SwiftShell-iOS.xcscheme
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "0830"
+   LastUpgradeVersion = "0900"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"
@@ -40,6 +40,7 @@
       buildConfiguration = "Debug-iOS"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      language = ""
       shouldUseLaunchSchemeArgsEnv = "YES">
       <Testables>
          <TestableReference
@@ -69,6 +70,7 @@
       buildConfiguration = "Debug-iOS"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      language = ""
       launchStyle = "0"
       useCustomWorkingDirectory = "NO"
       ignoresPersistentStateOnLaunch = "NO"
diff --git a/SwiftShell.xcodeproj/xcshareddata/xcschemes/SwiftShell.xcscheme b/SwiftShell.xcodeproj/xcshareddata/xcschemes/SwiftShell.xcscheme
index 329ce0b..0ea282d 100644
--- a/SwiftShell.xcodeproj/xcshareddata/xcschemes/SwiftShell.xcscheme
+++ b/SwiftShell.xcodeproj/xcshareddata/xcschemes/SwiftShell.xcscheme
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <Scheme
-   LastUpgradeVersion = "0830"
+   LastUpgradeVersion = "0900"
    version = "1.3">
    <BuildAction
       parallelizeBuildables = "YES"
@@ -54,6 +54,7 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      language = ""
       shouldUseLaunchSchemeArgsEnv = "YES"
       codeCoverageEnabled = "YES">
       <Testables>
@@ -104,6 +105,7 @@
       buildConfiguration = "Debug"
       selectedDebuggerIdentifier = "Xcode.DebuggerFoundation.Debugger.LLDB"
       selectedLauncherIdentifier = "Xcode.DebuggerFoundation.Launcher.LLDB"
+      language = ""
       launchStyle = "0"
       useCustomWorkingDirectory = "NO"
       ignoresPersistentStateOnLaunch = "NO"