Ruby Set(Set implements a collection of unordered values with no duplicates. This is a hybrid of Array's intuitive inter-operation facilities and Hash's fast lookup) (reference : Eloquent ruby)

#Usual way 

unique = []
words.each { |word| unique << word unless unique.include?(word) }

#What we really need is a collection that doesn’t allow duplicates but does feature very fast and easy answers to the “is this 
#object in there?” question. The punch line is that we need a set. Fortunately, Ruby comes with a perfectly serviceable, if 
#sometimes forgotten, 

#Set class:

require 'set'
word_set = Set.new( words )

#methods

s1.subset? s2                     
s2.subset? s1