EMU/Halle Library - sampling of philosophy resources on the Internet. Click on this link and try these searches: Arts & Humanities Search; JSTOR; Philosopher's Index; Project Muse.
This program lets you check proofs on-line (no special software needed!) or download DOS/Windows versions. Note that the arrow (->) is an alternative symbol for the horseshoe, and the double arrow (<->) is an alternative for the triple-bar.