A development of the theory of postulated colimits in Coq. Includes a proof that the pushout of a monomorphism is a monomorphism in a 'sheaf topos'.
Install Coq here.
If developing in emacs place the following lines:
(setq coq-load-path-include-current t)
(setq coq-compile-before-require t)
in your init file. If not then you may have to run coqc on the various dependencies to run the proofs.