Control inf recursion on degenerate labels

[linas]

The following graph

        +---------------------------Xp--------------+
        +--------------------WV---------------+     |
        |            +------------S-----------+     |
        |            +------MXs------+        |     |
        |            |  +------Xd----+        |     |
        +-----W------+  | +---Ds-----+        |     |
        |      +--D--+  | |    +--A--+-Xc-+   +--Xc-+
        |      |     |  | |    |     |    |   |     |
    LEFT-WALL the   dog , a black   lab   , barked  .

allows infinite recursion, because the Xc to period can attach to lab, which opens up a new, unconnected LEFT-WALL, which allows the graph to be further expanded, ad infinitum.

One obvious fix is to allow only one LEFT-WALL per graph. Another is the force planar graphs (issue #5) Are there other, more interesting solutions?