Foreach decrementation

Since you didn't specify any use case or requirements, I've gone with the laziest approach I can think of to accomplish this programatically:

reverse(a!forEach(enumerate(5), fv!index))

Another approach to this except what Mike mentioned can be - 

a!forEach(
  items: enumerate(5),
  expression: 5-fv!item
)

There can be endless ways of achieving a solution to a problem. But if you specify your use case and explain the problem better, you might find the most accurate ways.