public enum Diff: CustomDebugStringConvertible, CustomDocConvertible, Equatable {
	case Empty
	case Patch(Fix, Fix)
	indirect case Copy(Syntax<Diff>)

	public var doc: Doc {
		switch self {
		case .Empty:
			return .Empty
		case let .Patch(a, b):
			return .Horizontal([
				.Wrap(Doc.Text("{-"), Doc(a), Doc.Text("-}")),
				.Wrap(Doc.Text("{+"), Doc(b), Doc.Text("+}"))
			])
		case let .Copy(a):
			return a.doc
		}
	}

	public var debugDescription: String {
		switch self {
		case .Empty:
			return ".Empty"
		case let .Patch(a, b):
			return ".Patch(\(a), \(b))"
		case let .Copy(a):
			return ".Copy(\(a))"
		}
	}

	public init(_ a: Fix, _ b: Fix) {
		switch (a, b) {
		case (.Empty, .Empty):
			self = .Empty

		case let (.Roll(a), .Roll(b)):
			switch (a, b) {
			case let (.Apply(a, aa), .Apply(b, bb)):
				// fixme: SES
				self = .Copy(.Apply(Diff(a, b), Array(zip(aa, bb).lazy.map(Diff.init))))

			case let (.Abstract(p1, b1), .Abstract(p2, b2)):
				self = .Copy(.Abstract(Array(zip(p1, p2).lazy.map(Diff.init)), Diff(b1, b2)))

			case let (.Assign(n1, v1), .Assign(n2, v2)) where n1 == n2:
				self = .Copy(.Assign(n2, Diff(v1, v2)))

			case let (.Variable(n1), .Variable(n2)) where n1 == n2:
				self = .Copy(.Variable(n2))

			case let (.Literal(v1), .Literal(v2)) where v1 == v2:
				self = .Copy(.Literal(v2))

			case let (.Group(n1, v1), .Group(n2, v2)):
				self = .Copy(.Group(Diff(n1, n2), Array(zip(v1, v2).lazy.map(Diff.init))))

			default:
				self = .Patch(Fix(a), Fix(b))
			}

		default:
			self = Patch(a, b)
		}
	}
}