+---+   +---+   +---+   +---+   +---+
x1       |   |   |   |   |   |   |   |   |   |
      ---+   +---+   +---+   +---+   +---+   +---

          +----+    +----+    +----+    +----+
x2        |    |    |    |    |    |    |    |
      ----+    +----+    +----+    +----+    +---

          +--+      ++   +---+   +-+     +---+
          |  |      ||   |   |   | |     |   |
x1+x2    ++  +-+ +--++---+   ----+ +-+  ++   |
         |     | |                   |  |    |
      ---+     +-+                   +--+    +---