Without being able to call pure functions within pure functions, what use is this beyond the trivial simple examples? Couldn't it be possible to pass a "this" argument as the current JSON node and have it traverse the tree or something? For example:

  {
    "math": {
      "sin": function(x) {...}
    },
    "numerics": {
      "sin2": function(x) {
        return this.parent.math.sin(x)
      },
      "parent": [automatically inserted by parser]
    }
  }