Added Metacircular Evaluator for Source§2 Lazy

src/evaluators/__tests__/source-2-lazy.test1.js

@@ -0,0 +1,2 @@
+parse_and_evaluate("function f(a,b) { return a === 1 ? a : b; } f(1, head(null));");
+// 1

src/evaluators/__tests__/source-2-lazy.test2.js

@@ -0,0 +1,21 @@
+parse_and_eval("function at(xs, n) {\
+   return n === 0\
+       ?  head(xs)\
+       :  at(tail(xs), n-1);\
+}\
+function intsfrom(n) {\
+   return pair(n, intsfrom(n+1));\
+}\
+function zipWith(f, xs, ys) {\
+   return xs === null\
+       ?  null\
+       :  ys === null\
+       ?  null\
+       :  pair(f(head(xs), head(ys)), zipWith(f, tail(xs), tail(ys)));\
+}\
+const facs = pair(1, zipWith((x, y) => x * y, intsfrom(1), facs));\
+function fac(n) {\
+   return at(facs, n);\
+}\
+fac(6);");
+// 720

src/evaluators/__tests__/source-2-lazy.test3.js

@@ -0,0 +1,9 @@
+const list_func = "function list (x,y) {\
+                    return pair (x,pair (y,null));}";
+const map_func = "function map (f,xs) {\
+                    return xs === null ? xs\
+                    : pair (f(head(xs)), map (f,tail (xs)));\
+                    }";
+const test2 = map_func + "const a = pair(2,a); const b = map (x => x * x, a);head(tail(b));";
+parse_and_eval(test2);
+// 4