#comparable operator <=>
# natural ordering
# 1 2 3 4...
# a b c
puts 1 <=> 1
puts 2 <=> 1
puts 1 <=> 2
p [2,3,1].sort
puts
puts "a" <=> "a"
puts "b" <=> "a"
puts "a" <=> "b"
p ["c","b","a"].sort