code: function fieldPicker(fields)
  {
    var map = {};
  
    if (fields instanceof Array) {
      fields.forEach(function(f) { map[f] = f; });
    } else if (typeof fields === 'object') {
      for (var key in fields) {
        map[key] = fields[key];
      }
    }
  
    return function(input, output) {
      var target = output || input;
      for (var key in target) {
        if (map[key] === undefined)
        delete target[key];
      }
    };
  }
docs: Map fields from a provider exclusively