class Gem::Resolver

def self.compose_sets(*sets)
  sets.compact!
  sets = sets.flat_map do |set|
    case set
    when Gem::Resolver::BestSet then
      set
    when Gem::Resolver::ComposedSet then
      set.sets
    else
      set
    end
  end
  case sets.length
  when 0 then
    raise ArgumentError, "one set in the composition must be non-nil"
  when 1 then
    sets.first
  else
    Gem::Resolver::ComposedSet.new(*sets)
  end
end