Collection#union
----------------
 
 # q1 OR q2
 collection = Person.all(:name => 'Ted Han') | Person.all(:name => 'Dan Kubb')
 
 collection.entries  # => SELECT * FROM people WHERE name = 'Ted Han' OR name = 'Dan Kubb'
 
 
Collection#intersection
-----------------------
 
 # q1 AND q2
 collection = Person.all(:name => 'Ted Han') & Person.all(:id => 1)
 
 collection.entries  # => SELECT * FROM people WHERE name = 'Ted Han' AND id = 1
 
 
Collection#difference
---------------------
 
 # q1 AND NOT(q2)
 collection = Person.all - Person.all(:name => 'Dan Kubb')
 
 collection.entries  # => SELECT * FROM people WHERE NOT(name = 'Dan Kubb')
 
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This would also have to work with Array and Set, eg:
 
  Person.all - [ Person.get(1), Person.get(2) ]
 
  # => SELECT * FROM people WHERE NOT(id IN(1, 2))
 
It could also be made to work with a Range although I am unsure if this is useful, eg:
 
  Person.all - Person.get(1)..Person.get(10)
 
  # => SELECT * FROM people WHERE NOT(id BETWEEN 1 AND 10)