[mizar] Requirement articles

[Josef Urban <urban@ktilinux.ms.mff.cuni.cz>]

Hi,

since the Library Committee is just working hard and silently, I have only 
now noticed that they added articles explaining all existing 
requirement directives some time ago.
Thanks very much for this - it makes translations of Mizar proofs to other 
formats easier, and also should be useful for users who want to know why 
the checker behaves the way it behaves.

These are articles boole, subset, numerals and real, all containing normal 
proofs, which also adds to the reliability of the methods used by the 
checker.

Best,
Josef Urban