README.md in trackler-2.2.1.27

- old
+ new
@@ -18,9 +18,53 @@
 whatever you get.
 
 Note that these series are only required to occupy *adjacent positions*
 in the input; the digits need not be *numerically consecutive*.
 
+## Implementation
+
+Define two functions: (Two? Yes, sometimes we ask more out of Go.)
+
+All returns a list of all substrings of s with length n.
+
+`All(n int, s string) []string`
+
+UnsafeFirst returns the first substring of s with length n.
+
+`UnsafeFirst(n int, s string) string`
+
+At this point you could consider this exercise complete and move on.
+
+But wait, maybe you ask a reasonable question: Why is the function
+called **Unsafe** First?  If you are interested, read on for a bonus
+exercise.
+
+### Bonus exercise:
+
+Once you get `go test` passing, try `go test -tags asktoomuch`.  This
+uses a *build tag* to enable a test that wasn't enabled before. Build
+tags allow for the selection of files to be used by the package. In
+our case it will control which test files are used. You can read more
+about those at
+[the Go documentation](https://golang.org/pkg/go/build/#hdr-Build_Constraints).
+
+You may notice that you can't make this *asktoomuch* test happy. We
+need a way to signal that in some cases you can't take the first N
+characters of the string. UnsafeFirst can't do that since it only
+returns a string.
+
+To fix that, let's add another return value to the function.  Define
+
+`First(int, string) (first string, ok bool)`
+
+and test with `go test -tags first`.
+
+The `ok bool` second return argument is a common and idiomatic pattern
+in Go. For example you see it in
+[Map lookups](https://blog.golang.org/go-maps-in-action) and
+[type assertions](https://tour.golang.org/methods/15).
+
+
 ## Running the tests
 
 To run the tests run the command `go test` from within the exercise directory.
 
 If the test suite contains benchmarks, you can run these with the `-bench`