Wal-Mart has joined a growing list of retailers issuing calls for a few good startups to help solve some sticky technology problems and create more compelling shopping experiences. The world’s largest retailer is hosting an open call, inviting startups to its Bentonville, Ark. headquarters to pitch to top executives. The program is being led by Wal-Mart’s Lab 415-C, the company’s technology division. It’s partnering with the Northwest Arkansas Technology Summit and has room for roughly 250 applicants.