question: public void addMapping ( Matcher < Request > requestMatcher , ResponseSource responseSource ) { mappings . addFirst ( new MatcherResponseSourcePair ( requestMatcher , responseSource ) ) ; }
target:  Configure a response for matching requests. Each time a request is sent that matches the supplied matcher, the to-be-specified Response or ResponseSource will be used. Request to response mappings defined using this method will be applied in reverse order of addition, meaning that you can override previous choices in test code.