code: function(expression, options) {
      if (options && options.isSetter) {
        return expressions.parseSetter(expression, this.globals, this.formatters, options.extraArgs);
      } else if (options && options.extraArgs) {
        var allArgs = [expression, this.globals, this.formatters].concat(options.extraArgs);
        return expressions.parse.apply(expressions, allArgs);
      } else {
        return expressions.parse(expression, this.globals, this.formatters);
      }
    }
docs: Parses an expression into a function using the globals and formatters objects associated with this instance of
  observations.
  @param {String} expression The expression string to parse into a function
  @param {Object} options Additional options to pass to the parser.
  `{ isSetter: true }` will make this expression a setter that accepts a value.
  `{ extraArgs: [ 'argName' ]` will make extra arguments to pass in to the function.
  @return {Function} A function that may be called to execute the expression (call it against a context using=
  `func.call(context)` in order to get the data from the context correct)