theorem Th43:
  (u+v)+w = u+(v+w)
proof
  set a = the Element of M;
  consider b such that
A1: u = vect(a,b) by Th35;
  consider c such that
A2: v = vect(b,c) by Th35;
  consider d such that
A3: w = vect(c,d) by Th35;
  (u+v)+w = vect(a,c)+w by A1,A2,Th40
    .= vect(a,d) by A3,Th40
    .= vect(a,b)+vect(b,d) by Th40
    .= u+(v+w) by A1,A2,A3,Th40;
  hence thesis;
end;
