Flatten array without Ruby#flatten

gistfile1.txt

def my_flat(a)
  arr = []
  if a.instance_of? (Array)
    a.each do |b|
      if b.instance_of? (Array)
        arr += b.map {|c| c }
      else
        arr << b
      end
    end
  else
    arr << a
  end
  puts arr
end

arr1 = [1,2,3]
arr2 = [[4,5],6]
a = [arr1, arr2, 7, [8,9],10 ,11, 12, [13, 14, 15, 16]]

my_flat(a)