## a
def get_bars(foos=[])
  bars = []

  foos.each do |foo|
    bars = bars | foo.bars
  end

  bars
end

## b
def get_bars(foos=[])
  foos.map(&:bars).inject{|a, b| a | b}
end

## c
def get_bars(foos=[])
  foos.map(&:bars).flatten.uniq
end