+---+
| C |
+---+
'
'
'
+---+ ' +---+
| A | ------+-----> | B |
+---+ ' +---+
'
'
v
+---+
| D |
+---+
+---+
| C |
+---+
'
'
'
+---+ ' +---+
| A | ------+-----> | B |
+---+ ' +---+
'
'
v
+---+
| D |
+---+