  Libraries of formal proofs are an important part of our mathematical
heritage, but their usability and sustainability is poor. Indeed, each library
is specific to a proof system, sometimes even to some version of this system.
Thus, a library developed in one system cannot, in general, be used in another
and when the system is no more maintained, the library may be lost. This
impossibility of using a proof developed in one system in another has been
noted for long and a remediation has been proposed: as we have empirical
evidence that most of the formal proofs developed in one of these systems can
also be developed in another, we can develop a standard language, in which
these proofs can be translated, and then used in all systems supporting this
standard. Logipedia is an attempt to build such a multi-system online
encyclopedia of formal proofs expressed in such as standard language. It is
based on two main ideas: the use of a logical framework and of reverse
mathematics.
