reserve x,y,z for object,
  i,j,n,m for Nat,
  D for non empty set,
  s,t for FinSequence,
  a,a1,a2,b1,b2,d for Element of D,
  p, p1,p2,q,r for FinSequence of D;

theorem
  <*<*a1*>,<*a2*>*> is Matrix of 2,1,D
proof
  len <*a1*>=1 & len <*a2*>=1 by FINSEQ_1:39;
  hence thesis by Th12;
end;
