function _validateScope(scope) {
    for (var symbol in scope) {
      if (hasOwnProperty(scope, symbol)) {
        if (symbol in keywords) {
          throw new Error('Scope contains an illegal symbol, "' + symbol + '" is a reserved keyword');
        }
      }
    }
  }
Validate the symbol names of a scope.
Throws an error when the scope contains an illegal symbol.
@param {Object} scope