class Concurrent::ThreadSafe::Util::Adder

incorporated.
occur while the sum is being calculated might not be
updates returns an accurate result, but concurrent updates that
atomic snapshot: Invocation in the absence of concurrent
Returns the current sum. The returned value is _NOT_ an

def sum
  x = base
  if current_cells = cells
    current_cells.each do |cell|
      x += cell.value if cell
    end
  end
  x
end