Fun project. From examining the source code, they are using the technique that can be considered a kind of higher order abstract syntax (HOAS). In particular, LJSON can be fooled by so called exotic terms. E.g.

  LJSON.stringify(function(v) { return v; })

evaluates to the string

  (function(v0){return v0})

While

  LJSON.stringify(function(v) { return v(null) } )

Evaluates to the string

  (function(v0){return "v0"})

In the latter case, v(null) was able to examine the variable that stringify injects into the function. In this case, v(null) is the name of the injected variable.