Abassi Walker stares up at the 13-foot tall vertical wall, a gray mass pocked with multicolored knobs, handholds and what amount to nothing more than slight bulges. Taking a deep breath, he grasps a handhold, places his right foot on one of the large knobs and pushes off. Long-limbed and lanky, he looks like a spider as he slowly makes his way up. Then a foot slips. Down…