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 Th19:
  <*<*a1,a2*>,<*b1,b2*>*> is Matrix of 2,D
proof
A1: len <*a1,a2*>=2 & len <*b1,b2*>=2 by FINSEQ_1:44;
  thus thesis by A1,Th12;
end;
