range.rb

class Range
  # Allows you to do the following
  #  
  #  a = (0..3)
  #  b = (2..4)
  #  a.intersection(b) #=> (2..3)
  #
  def intersection(other)
    if self.max < other.begin || other.max < self.begin
      return (1..0)
    end
    
    first = [self.begin, other.begin].max 
    last  = [self.max, other.max].min 
    (first..last)
  end
  alias_method :&, :intersection
end

Returning [] is also invalid because what if someone wants a Range, not an Array. Returning an empty array breaks the contract.

I agree with Percy.