diff --git a/prototype/Doubt.xcodeproj/project.pbxproj b/prototype/Doubt.xcodeproj/project.pbxproj
index 314c4060a..09e3336b6 100644
--- a/prototype/Doubt.xcodeproj/project.pbxproj
+++ b/prototype/Doubt.xcodeproj/project.pbxproj
@@ -46,6 +46,7 @@
 		D4C2B1421BB3474C0096F92A /* SwiftXPC.framework in Copy Frameworks */ = {isa = PBXBuildFile; fileRef = D4413FD61BB0531E00E3C3C1 /* SwiftXPC.framework */; settings = {ATTRIBUTES = (CodeSignOnCopy, RemoveHeadersOnCopy, ); }; };
 		D4C2B1431BB3474C0096F92A /* SourceKittenFramework.framework in Copy Frameworks */ = {isa = PBXBuildFile; fileRef = D4413FD41BB052ED00E3C3C1 /* SourceKittenFramework.framework */; settings = {ATTRIBUTES = (CodeSignOnCopy, RemoveHeadersOnCopy, ); }; };
 		D4C2B1441BB347500096F92A /* Doubt.framework in Copy Frameworks */ = {isa = PBXBuildFile; fileRef = D4AAE4FD1B5AE22E004E581F /* Doubt.framework */; settings = {ATTRIBUTES = (CodeSignOnCopy, RemoveHeadersOnCopy, ); }; };
+		D4D7F3171BBB22E500AAB0C0 /* Hash.swift in Sources */ = {isa = PBXBuildFile; fileRef = D4D7F3161BBB22E500AAB0C0 /* Hash.swift */; settings = {ASSET_TAGS = (); }; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -109,6 +110,7 @@
 		D4AAE53E1B5AE2D0004E581F /* StringLiteralConvertible.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = StringLiteralConvertible.swift; sourceTree = "<group>"; };
 		D4AAE53F1B5AE2D0004E581F /* Syntax.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Syntax.swift; sourceTree = "<group>"; };
 		D4AAE54B1B5AE42D004E581F /* Equatable.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Equatable.swift; sourceTree = "<group>"; };
+		D4D7F3161BBB22E500AAB0C0 /* Hash.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = Hash.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -196,6 +198,7 @@
 				D4413FEE1BB06D4C00E3C3C1 /* Dictionary.swift */,
 				D4413FF01BB08FDC00E3C3C1 /* JSON.swift */,
 				D4A71DC41BB45B850051416D /* Vertex.swift */,
+				D4D7F3161BBB22E500AAB0C0 /* Hash.swift */,
 				D4AAE5001B5AE22E004E581F /* Supporting Files */,
 			);
 			path = Doubt;
@@ -375,6 +378,7 @@
 				D4413FEF1BB06D4C00E3C3C1 /* Dictionary.swift in Sources */,
 				D4AAE5481B5AE2D0004E581F /* String.swift in Sources */,
 				D4AAE5411B5AE2D0004E581F /* Diff.swift in Sources */,
+				D4D7F3171BBB22E500AAB0C0 /* Hash.swift in Sources */,
 				D4AAE5401B5AE2D0004E581F /* Array.swift in Sources */,
 				D4AAE54C1B5AE42D004E581F /* Equatable.swift in Sources */,
 				D4A71DC51BB45B850051416D /* Vertex.swift in Sources */,
diff --git a/prototype/Doubt/Hash.swift b/prototype/Doubt/Hash.swift
new file mode 100644
index 000000000..462b2f9f7
--- /dev/null
+++ b/prototype/Doubt/Hash.swift
@@ -0,0 +1,78 @@
+public enum Hash: Hashable {
+	case Sequence([Hash])
+	case String(Swift.String)
+	case Int(Swift.Int)
+
+	public static func Case(label: Swift.String, _ hashes: Hash...) -> Hash {
+		return .Sequence([ .String(label) ] + hashes)
+	}
+
+	public static func Case(index: Swift.Int, _ hashes: Hash...) -> Hash {
+		return .Sequence([ .Int(index) ] + hashes)
+	}
+
+	public init<A: AlgebraicHashable>(_ hashable: A) {
+		self = hashable.hash
+	}
+
+	public init<A: Hashable>(_ hashable: A) {
+		self = .Int(hashable.hashValue)
+	}
+
+
+	public var hashValue: Swift.Int {
+		switch self {
+		case let .Sequence(s):
+			// Bob Jenkins’ one-at-a-time hash: https://en.wikipedia.org/wiki/Jenkins_hash_function
+			var hash = 0
+			for each in s {
+				hash += each.hashValue
+				hash += hash << 10
+				hash ^= hash >> 6
+			}
+			hash += hash << 3
+			hash ^= hash >> 11
+			hash += hash << 15
+			return hash
+		case let .String(s):
+			return s.hashValue
+		case let .Int(i):
+			return i.hashValue
+		}
+	}
+}
+
+public func == (left: Hash, right: Hash) -> Bool {
+	switch (left, right) {
+	case let (.Sequence(a), .Sequence(b)):
+		return a == b
+	case let (.String(a), .String(b)):
+		return a == b
+	case let (.Int(a), .Int(b)):
+		return a == b
+	default:
+		return false
+	}
+}
+
+public protocol AlgebraicHashable: Hashable {
+	var hash: Hash { get }
+}
+
+extension AlgebraicHashable {
+	public var hashValue: Int {
+		return hash.hashValue
+	}
+}
+
+extension RawRepresentable where RawValue: Hashable {
+	public var hash: Hash {
+		return Hash(rawValue)
+	}
+}
+
+extension RawRepresentable where RawValue: AlgebraicHashable {
+	public var hash: Hash {
+		return Hash(rawValue)
+	}
+}
diff --git a/prototype/Doubt/Syntax.swift b/prototype/Doubt/Syntax.swift
index 9ffff0cb0..8c83c3dee 100644
--- a/prototype/Doubt/Syntax.swift
+++ b/prototype/Doubt/Syntax.swift
@@ -1,4 +1,4 @@
-public enum Term: CustomDebugStringConvertible, CustomDocConvertible, CustomStringConvertible, Equatable {
+public enum Term: CustomDebugStringConvertible, CustomDocConvertible, CustomStringConvertible, AlgebraicHashable {
 	public init(_ out: Syntax<Term>) {
 		self = .Roll(out)
 	}
@@ -27,6 +27,11 @@ public enum Term: CustomDebugStringConvertible, CustomDocConvertible, CustomStri
 	}
 
 
+	public var hash: Hash {
+		return syntax.hash
+	}
+
+
 	public static var Empty: Term {
 		return Term(.Empty)
 	}
@@ -180,7 +185,8 @@ public enum Syntax<Payload>: CustomDebugStringConvertible, CustomDocConvertible
 			return ".Apply(\(f), [ \(s) ])"
 		case let .Abstract(parameters, body):
 			let s = parameters.map { String(reflecting: $0) }.joinWithSeparator(", ")
-			return ".Abstract([ \(s) ], \(body))"
+			let b = body.map { String(reflecting: $0) }.joinWithSeparator("\n")
+			return ".Abstract([ \(s) ], \(b))"
 		case let .Assign(n, v):
 			return ".Assign(\(n), \(v))"
 		case let .Variable(n):
@@ -215,3 +221,24 @@ public enum Syntax<Payload>: CustomDebugStringConvertible, CustomDocConvertible
 		}
 	}
 }
+
+extension Syntax where Payload: AlgebraicHashable {
+	public var hash: Hash {
+		switch self {
+		case .Empty:
+			return .Case("Empty")
+		case let .Apply(f, vs):
+			return .Case("Apply", .Sequence([ f.hash ] + vs.map { $0.hash }))
+		case let .Abstract(parameters, body):
+			return .Case("Abstract", .Sequence(parameters.map { $0.hash }), .Sequence(body.map { $0.hash }))
+		case let .Assign(name, value):
+			return .Case("Assign", .String(name), value.hash)
+		case let .Variable(n):
+			return .Case("Variable", .String(n))
+		case let .Literal(s):
+			return .Case("Literal", .String(s))
+		case let .Group(n, vs):
+			return .Case("Group", n.hash, .Sequence(vs.map { $0.hash }))
+		}
+	}
+}