How to iterate for loop in reverse order in swift?

When I use the for loop in Playground, everything worked fine, until I changed the first parameter of for loop to be the highest value. (iterated in descending order)

Is this a bug? Did any one else have it?

for index in 510..509
{
    var a = 10
}

The counter that displays the number of iterations that will be executions keeps ticking…