hailstone := proc( N )
    local n := N, HS := Array([n]);
    while n > 1 do
        if type(n,even) then
            n := n/2;
        else
            n := 3*n+1;
        end if;
        HS(numelems(HS)+1) := n;
    end do;
    HS;
end proc;