Fixed missing detection of adding element. Now it's supposed to be the last generator.

Still one bug:
testex:=NewSphereMachine("a=<b*c*b^-1, d>","c=<d^-1,d>(1,2)","d=<a,1>","b=<b,1>(1,2)","a*b*c*d");
ThurstonAlgorithm(testex);
causes an error "c.cell not available" (c=fail)