selecting highest numbered object Oracle 12c

Need to select the slip with the largest number when grouped by stock from objects below:

create table tab1(horse number, stock number);
insert into tab1 values(100,4);
insert into tab1 values(101,5);

create table tab2(stock number, slip number);
insert into tab2 values(4, 1);
insert into tab2 values(4, 5);
insert into tab2 values(4, 8);
insert into tab2 values(5, 1);
insert into tab2 values(5, 5);
insert into tab2 values(5, 6);
insert into tab2 values(5, 7);
commit;

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desired results
horse  stock  slip
 100       4        8
 101       5        7
 

 select t1.horse, t2.stock, t2.slip,
   rank ()
                   over (partition by horse, slip
                         order by slip desc)
                       rnk
  from tab1 t1
  join tab2 t2
  on t1.stock = t2.stock
  where rnk = 1

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returns error:  "rnk" invalid identifier....

all pointers & suggestions appreciated...