European Factors to Watch-Shares seen edging lower

LONDON, May 17 (Reuters) - European stocks were expected to edge lower on Friday, with comments from U.S. central bank officials saying the Fed could begin easing up its stimulus this summer prompting investors to trim their trading positions. John Williams, president of the Federal Reserve Bank of San Francisco, said on Thursday the Fed could end bond purchases later this year, assuming the labour market continues to strengthen, while Dallas Fed President Richard Fisher said the Fed should