I have a `SizeMatters` class that creates an object from a given string. In order to sort these objects in an array, I've implemented the `<=>(other)` method. But the following code only helps the objects to be sorted by size. I also want the array to be sorted alphabetically. 

```
class SizeMatters
  include Comparable
  attr :str
  def <=>(other)
    str.size <=> other.str.size
  end
  def initialize(str)
    @str = str
  end
  def inspect
    @str
  end
end

s1 = SizeMatters.new("Z")
s2 = SizeMatters.new("YY")
s3 = SizeMatters.new("xXX")
s4 = SizeMatters.new("aaa")
s5 = SizeMatters.new("bbb")
s6 = SizeMatters.new("WWWW")
s7 = SizeMatters.new("VVVVV")

[ s3, s2, s5, s4, s1 , s6, s7].sort #[Z, YY, bbb, xXX, aaa, WWWW, VVVVV]

```

What I want is this

```
[ s3, s2, s5, s4, s1 , s6, s7].sort #[Z, YY, aaa, bbb, xXX, WWWW, VVVVV]

```

How can I write `<=>(other)` so that objects in array can be sorted first by size and then alphabetically?