Add TopFrame function

pperr.go

@@ -97,3 +97,23 @@ func CauseType(err error) string {
 		err = cause
 	}
 }
+
+func TopFrame(es []ErrorSet) (*Frame, error) {
+	if len(es) == 0 {
+		return nil, fmt.Errorf("length of es greater than 0")
+	} else if len(es[0].Frames) == 0 {
+		return nil, fmt.Errorf("length of es[0].Frames greater than 0")
+	}
+
+	top := es[0].Frames
+
+	if p := es[0].Parent; p != nil {
+		top = top.Exclude(p)
+	}
+
+	if len(top) == 0 {
+		return nil, fmt.Errorf("unique top frame not found")
+	}
+
+	return top[0], nil
+}