reorganize directory and file structure

lib/structures/product.js

@@ -0,0 +1,9 @@
+// Product-type (also called: pair, tuple)
+
+// A Product-type always has only two fields, called "left" and "right".
+// Product-types of more fields (called Structs) can be constructed by nesting Product-types.
+
+// In JS, all products are encoded in the same way:
+export const newProduct = l => r => ({l, r});
+export const getLeft     = product => product.l;
+export const getRight    = product => product.r;