question: final public MutableString append ( MutableString s ) { if ( s == null ) s = NULL ; final int l = s . length ( ) ; if ( l == 0 ) return this ; final int newLength = length ( ) + l ; expand ( newLength ) ; System . arraycopy ( s . array , 0 , array , newLength - l , l ) ; hashLength = hashLength < 0 ? - 1 : newLength ; return this ; }
target:  Appends the given mutable string to this mutable string.