  The data model of an application, the nature and format of data stored across
executions, is typically a very rigid part of its early specification, even
when prototyping, and changing it after code that relies on it was written can
prove quite expensive and error-prone.
  Code and data in a running Lisp image can be dynamically modified. A
MOP-based persistence library can bring this dynamicity to the data model. This
enables to extend the easy prototyping way of development to the storage of
data and helps avoiding interruptions of service. This article presents the
conditions to do this portably and transparently.
