def increment_mask_for column_name, *fields
      fields = bitfields if fields.empty?
      column = @@bitfields[column_name]
      raise ArgumentError, "Unknown column for bitfield: #{column_name}" if column.nil?
      fields.sum do |field_name|
        raise ArugmentError, "Unknown field: #{field_name} for column #{column_name}" if column[field_name].nil?
        2 ** (bitfield_max(column_name) - column[field_name].last)
      end
    end
def increment_mask_for column_name, *fields
      fields = bitfields if fields.empty?
      column = @@bitfields[column_name]
      raise ArgumentError, "Unknown column for bitfield: #{column_name}" if column.nil?
      fields.sum do |field_name|
        raise ArugmentError, "Unknown field: #{field_name} for column #{column_name}" if column[field_name].nil?
        2 ** (bitfield_max(column_name) - column[field_name].last)
      end
    end
Returns an "increment mask" for a list of fields

 +increment_mask_for :fields

 @example
  user.increment_mask_for :field

 @param [ Symbol ] column     name of the column these fields are in
 @param [ Symbol ] field(s) name of the field(s) for the mask