A 12 year old girl who had just arrived home from collecting Halloween candy was taken at gunpoint from the Pomona mobile home park where she lived wtih her mother and we sexually assaulted repeatedly over a six to eight hour period.  The man, William Robert Riley, 40, of West Covina, was sentenced to 936 years to life in prison.  He was convicted last month of 17 counts, including lewd acts with a child, kidnapping and making terroristic threats.  Riley had only been out of prison five days when he turned again to crime.  The judge, Superior Court Judge David S. Milton told him he should never be free again.  The girl lived in the US for only two weeks at the time of the attack and has since returned to Tijuana, Mexico to live with her father.  The girl's 5 year old sister was left bound and gagged in the mobile home during the attack but managed to free herself and alert police who chased and caught Riley after he returned the older girl.  Ten separate 25 years to life sentences and seven 75 years to life sentences, along with 15  10 year sentences for use of a gun and an 11 year sentence for Riley's prior felonies were imposed by Milton.  Deputy District Attorney Kim Santini said it could be one of the longest sentences for a sex offender in Los Angeles County history.