Add test coverage. (#4)

* Add test coverage.

* Add test coverage. Use Go multierrors instead of go.uber.org/multierr.

* More tests and some refactoring.

Readme.md

@@ -16,7 +16,7 @@ if it has a more-specific type than it actually needs.
 For example,
 if your function takes a `*os.File` parameter,
 but it’s only ever used for its `Read` method,
-it could be specified as an abstract `io.Reader` instead
+it could be specified as an abstract `io.Reader` instead.
 
 ## Why decouple?
 

_testdata/foo.go

@@ -101,7 +101,8 @@ func F12(f *os.File) ([]byte, error) {
 }
 
 // {"ctx": {"Done": "func() <-chan struct{}"},
-//  "r":   {"Read": "func([]byte) (int, error)"}}
+//
+//	"r":   {"Read": "func([]byte) (int, error)"}}
 func F13(ctx context.Context, ch chan<- io.Reader, r *os.File) {
 	for {
 		select {
@@ -337,7 +338,8 @@ func F41(w io.Writer, readers []io.Reader) error {
 }
 
 // {"ctx": {"Done": "func() <-chan struct{}", "Err": "func() error"},
-//  "f": {"Name": "func() string"}}
+//
+//	"f": {"Name": "func() string"}}
 func F42(ctx context.Context, f *os.File, ch <-chan struct{}) (string, error) {
 	select {
 	case <-ctx.Done():
@@ -359,3 +361,42 @@ func F43(w io.Writer, f *os.File) error {
 	}
 	return fn(f)
 }
+
+// {}
+func F44(s []int, x, y, z int) []int {
+	return s[(x+1)*2 : y : z] // exercises *ast.ParenExpr and *ast.SliceExpr
+}
+
+// {}
+func F45(m map[string]int, k string) int {
+	return m[k]
+}
+
+// {"f": {"Read": "func([]byte) (int, error)"}}
+func F46(f *os.File) ([]byte, error) {
+	fn := func(r io.Reader) ([]byte, error) {
+		return io.ReadAll(r)
+	}
+	return fn(f)
+}
+
+type t47 struct {
+	f *os.File
+	r io.Reader
+}
+
+// {}
+func F47(f *os.File) t47 {
+	return t47{f: f, r: f}
+}
+
+// {"f": {"Read": "func([]byte) (int, error)"}}
+func F48(f *os.File) t47 {
+	return t47{r: f}
+}
+
+// {}
+func F49(n int) int {
+	n++
+	return n
+}