def reset_mask_for column_name, *fields
      fields = bitfields if fields.empty?
      max = bitfield_max(column_name)
      (0..max).sum{|i| 2 ** i} - only_mask_for(column_name, *fields)
    end
def reset_mask_for column_name, *fields
      fields = bitfields if fields.empty?
      max = bitfield_max(column_name)
      (0..max).sum{|i| 2 ** i} - only_mask_for(column_name, *fields)
    end
Returns a "reset mask" for a list of fields

 +reset_mask_for :fields

 @example
  user.reset_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