examples/iface: expand test

Change-Id: Ibe437b2af33ec9adcf136e51acf50bbc5f4e18b7

_examples/iface/iface.go

@@ -2,8 +2,28 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// package iface tests various aspects of interfaces.
 package iface
 
+import (
+	"github.com/go-python/gopy/_examples/cpkg"
+)
+
+// Iface has a single F() method
 type Iface interface {
 	F()
 }
+
+// T implements Iface
+type T struct{}
+
+func (t *T) F() {
+	cpkg.Printf("t.F [CALLED]\n")
+}
+
+// CallIface calls F() on v
+func CallIface(v Iface) {
+	cpkg.Printf("iface.CallIface...\n")
+	v.F()
+	cpkg.Printf("iface.CallIface... [DONE]\n")
+}