irb(main):001:0> buff = Hash.new(0)
=> {}
irb(main):002:0> buff[:hello] += 1
=> 1
irb(main):003:0> buff[:there] += 4
=> 4
irb(main):004:0> buff
=> {:hello=>1, :there=>4}

line_items.inject({}) do |tax_groups, line_item|
        if tax_groups.key?(line_item["tax_lines"])
          tax_groups[key].push(line_item)
        else
          tax_groups[key] = [line_item]
        end

        tax_groups
      end

each.with_object(Hash.new(0)) do |k, o|
  o[k] += 1
end

each_with_object(Hash.new([])) do |line_item, tax_groups|
  tax_groups[key] << line_item
end

tax_groups[key] ||= []
tax_groups[key] << line_item