Fix is SyntaxConvertible.

prototype/Doubt/Fix.swift

@@ -1,7 +1,7 @@
 /// The fixpoint of `Syntax`.
 ///
 /// `Syntax` is a non-recursive type parameterized by the type of its child nodes. Instantiating it to `Fix` makes it into a recursive tree by “tying the knot”—each child node of `Syntax<Fix, A>` is represented by a `Fix` which in turn contains a `Syntax<Fix, A>`. So in the same way that the `fix` function allows one to tie a non-recursive function into a recursive one, `Fix` allows one to tie a non-recursive type into a recursive one. Unfortunately, due to Swift’s lack of higher-rank types, this cannot currently be abstracted over the type which is made recursive, and thus it is hard-coded to `Syntax<Fix, A>` rather than provided by a type parameter `F` applied to `Fix<F>`.
-public enum Fix<A>: CustomDebugStringConvertible, CustomDocConvertible {
+public enum Fix<A>: CustomDebugStringConvertible, CustomDocConvertible, SyntaxConvertible {
 	/// A recursive instantiation of `Syntax`, unrolling another iteration of the recursive type.
 	indirect case In(Syntax<Fix, A>)
 
@@ -25,6 +25,13 @@ public enum Fix<A>: CustomDebugStringConvertible, CustomDocConvertible {
 	public var doc: Doc {
 		return out.doc
 	}
+
+
+	// MARK: SyntaxConvertible
+
+	public init(syntax: Syntax<Fix, A>) {
+		self = .In(syntax)
+	}
 }