# File lib/randexp/parser.rb, line 68
    def self.union(lhs, *rhs)
      if lhs.nil?
        union(*rhs)
      elsif rhs.empty?
        lhs
      elsif lhs.first == :union
        rhs.each {|s| lhs << s}
        lhs
      else
        [:union, lhs, *rhs]
      end
    end