 " [ a ] court ' must presume that a defendant did not waive his right [ s ] ' ; the prosecution bears a ' heavy burden ' in attempting to demonstrate waiver ; the fact of a ' lengthy interrogation ' prior to obtaining statements is ' strong evidence ' against a finding of valid waiver ; ' mere silence ' in response to questioning is ' not enough ' ; and waiver may not be presumed ' simply from the fact that a confession was in fact eventually obtained ' . " 
