detect (some) terms without a normal form.

src/reduce.mjs

@@ -1,12 +1,23 @@
+function serialize (expr) {
+	return Array.isArray(expr)
+		? "(" + expr.map(serialize).join(",") + ")"
+		: expr;
+}
+
 export function reduce (expr, max_steps = 1000) {
 	if (!is_application(expr)) {
 		return expr;
 	}
 
 	let result = reduce_single(expr);
+	const visited = new Map();
 
 	// trampoline
 	for (let step = 0; step < max_steps; step++) {
+		const key = serialize(result);
+
+		if (visited.has(key)) return result;
+		visited.set(key, true);
 		result = reduce_single(result);
 	}