question: default T fold ( final T identity , final BinaryOperator < T > accumulator ) { final Optional < T > opt = toOptional ( ) ; if ( opt . isPresent ( ) ) return accumulator . apply ( identity , get ( ) ) ; return identity ; }
target:  Use the supplied identity value and function to reduce this Value to a single result (unwraps the value stored in this Value  if the provided monoid combination instance obeys the Monoid laws)