Test internal/grammar, and allow the root to import it

internal/grammar/reference_test.go

@@ -0,0 +1,21 @@
+package grammar
+
+import "testing"
+
+func TestRefShapeSplitsTheSigil(t *testing.T) {
+	for ref, want := range map[string][2]string{
+		"/a.b":  {"/", "a.b"},
+		".a":    {".", "a"},
+		"..a.b": {"..", "a.b"},
+	} {
+		sigil, rest, err := RefShape(ref)
+		if err != nil || sigil != want[0] || rest != want[1] {
+			t.Errorf("RefShape(%q) = %q, %q, %v, want %q", ref, sigil, rest, err, want)
+		}
+	}
+	for _, ref := range []string{"/", "//a", ".../a", "/a..b.", "/a.[b]"} {
+		if _, _, err := RefShape(ref); err == nil {
+			t.Errorf("RefShape(%q) = nil error, want one", ref)
+		}
+	}
+}