question: public String toString ( String sep ) { if ( isEmpty ( ) ) { return "" ; } else { StringBuilder buf = new StringBuilder ( ) ; buf . append ( head ) ; for ( List < A > l = tail ; l . nonEmpty ( ) ; l = l . tail ) { buf . append ( sep ) ; buf . append ( l . head ) ; } return buf . toString ( ) ; } }
target:  Form a string listing all elements with given separator character.