recreate table t(id int, x int);
commit;
insert into t values(1, 100);
commit;

execute block returns(new_x int) as
  declare i int = 1;
  declare x int;
  declare c cursor for(select x from t where id = :i);
begin
  open c;
  while (1=1) do
  begin
    fetch c into x;
    if ( row_count = 0) then leave;
    update t set x = :x * 2
     where current of c 
    --where id = :i
    returning x into new_x;
    suspend;
  end
  close c;
endd

Invalid token.
Dynamic SQL Error.
SQL error code = -104.
Token unknown - line 14, column 5.
returning.

     where current of c 

    where id = :i

. . .
  close c;
end d 

set term ^;
execute block returns(new_x int) as
  declare i int = 1;
  declare x int;
  declare c cursor for(select x from t where id = :i);
begin
  open c;
  while (1=1) do
  begin
    fetch c into x;
    if ( row_count = 0) then leave;
    update t set x = :x * 2
    where current of c
    --where id = :i
    returning x into new_x;
    suspend;
  end
  close c;
end
^
set term ;^
rollback;