func Cons(i interface{}, l List) List {
	return Consf(func() interface{} { return i }, l)
}
Cons pushes an element onto the fron of the list
package list
type unit struct{}
type List interface{}
func Mzero() List {
	return unit{}
}
func Return(i interface{}) List {
	return Returnf(func() interface{} { return i })
}