Oh look: it's solitaire! Not surprising, really. Every computer system needs a solitaire game. Sort the cards by alternating suit colour? Check. Populate four foundation piles from ace to king? Check. It's basically Klondike without some of the usual options you might expect. You draw spare cards one at a time, but when the stack is exhausted: that's it, you're done.
As expected, this variant is quite hard.
Any face-down cards in the stacks are not displayed, but rather are directly underneath the face-up card and get displayed when the upper card is moved. This game is played with a typical drag + drop motion, though there is no graphical indication that you are moving a card until you release the mouse button.
XSol was originally written as solitaire.c by David W. Burleigh in April 1987, for the Workstation Sales Support Training (whatever that was). It was later updated a couple times by Dave Lemke in 1987 and 1988, then finally patched to be Linux compatible by someone with the initials of DLM in 1992. It has a copyright for Sun Microsystems, so I assume it was originally written for SunOS. However, the copyright is for 1988, so I have no idea if the original 1987 development was there or not.
The following is a typical session, in which I ran out of cards:
XSol can be downloaded from the SunSITE archives at ibiblio.org. I was only able to build this game on older Linux distributions. It requires X10 compatibility headers, which are not commonly found any more. If you wish to run on a later distro, some porting will be needed.
More general information on old X games, including build tips can be found in the Old X Games article.
If the above video does not work for you, it is also available on my YouTube channel.