code: function(scope, defaultValue) {
              var val = evaluator(tree, scope);
              if( typeof defaultValue === 'undefined') {
                  return val;
              } else {
                  return (val === undefined || val === null || val != val) ? defaultValue : val;
              }
          }
docs: Evaluates an expression against a scope
  @param scope
  @return {*} - value of an expression in a given scope