code: def delta(other_audit)
      
        return self.change_log if other_audit.nil?
      
        {}.tap do |d|
          
          # first for keys present only in this audit
          (self.change_log.keys - other_audit.change_log.keys).each do |k|
            d[k] = [nil, self.change_log[k]]
          end
      
          # .. then for keys present only in other audit
          (other_audit.change_log.keys - self.change_log.keys).each do |k|
            d[k] = [other_audit.change_log[k], nil]
          end
      
          # .. finally for keys present in both, but with different values
          self.change_log.keys.each do |k|
            if self.change_log[k] != other_audit.change_log[k]
              d[k] = [other_audit.change_log[k], self.change_log[k]]
            end
          end
      
        end
  
      end
docs: Computes the differences of the change logs between two audits.
  
   Returns a hash containing arrays of the form
     {
       :key_1 => [<value_in_other_audit>, <value_in_this_audit>],
       :key_2 => [<value_in_other_audit>, <value_in_this_audit>],
     }