theorem Th13:
  w1^v1 = w2^v2 implies (ex u st w1^u = w2 & v1 = u^v2) or ex u st
  w2^u = w1 & v2 = u^v1
proof
A1: len w1 < len w2 or len w1 >= len w2;
  assume w1^v1 = w2^v2;
  hence thesis by A1,Th12;
end;
