gistfile1.txt

pry(main):12> ab = 1                                                                                                   
=> 1
pry(main):13> c = 2                                                                                                    
=> 2
pry(main):14> abc = Set.new [ab, c]                                                                                    
=> #<Set: {1, 2}>
pry(main):15> abc > c                                                                                                  
ArgumentError: value must be a set
  from /usr/lib/ruby/2.2.0/set.rb:226:in `proper_superset?'
pry(main):16> abc > Set.new([c])                                                                                       
=> true
pry(main):17> abc - c                                                                                                  
ArgumentError: value must be enumerable
  from /usr/lib/ruby/2.2.0/set.rb:98:in `do_with_enum'
pry(main):18> abc - Set.new([c])                                                                                       
=> #<Set: {1}>