Two queries concerning proof checkers have appeared. How about either
(1) The Boyer-Moore theorem prover.
Contact
Computational Logic Inc
1717 West Sixth St Suite 290
Austin Texas 78703
(2) Mizar.
Contact
Andrzej Trybulec / Howard Blair
EECS
University of Connecticut
Storrs CT 06268
Leslie Burkholder
(If anyone suggests anything else please let me know.)