gistfile1.rb

...
 cell.recalculate
...

class Sheet
  def build_sheet 
    ...
    cell = ValueCell.new(self,"A1","A2")
    ...
    cell.recalculate
    ...
  end
end

class Sheet
  def build_sheet(cell) 
    ...
    cell.recalculate
    ...
  end
end