Set membership

[explosiveamber]

As the name goes. This is a totally awesome package and I've found it very beneficial. Dropping by some ideas to bring its UnicodeMath::Set module to a greater functionality, hth:

        define_method('∈') do |elem|
         self.include?(elem)
        end

        define_method('∉') do |elem|
          !(self.include?(elem))
        end

        define_method('∅') do
          self.empty?
        end

set_spec.rb

 it 'defines the membership operator: ∈'  do
     membership = @primes.∈(2)
     expect(membership).to eq(true)   
   end

   it 'defines the non-membership operator: ∉'  do
     nonmembership = @primes.∉(6)
     expect(nonmembership).to eq(true)    
   end

   it 'defines the empty set operator: ∅'  do
     empty_set = @empty_set.∅
     expect(empty_set).to eq(true)    
   end

[tbugai]

Can you put these into a pull requests?