reserve i,j,m,n,k for Nat,
  x,y for set,
  K for Field,
  a,a1,a2 for Element of K,
  D for non empty set,
  d,d1,d2 for Element of D,
  M,M1,M2 for (Matrix of D),
  A,A1,A2,B1,B2 for (Matrix of K),
  f,g for FinSequence of NAT;
reserve F,F1,F2 for FinSequence_of_Matrix of D,
  G,G9,G1,G2 for FinSequence_of_Matrix of K;

theorem Th16:
  Sum Len <*M1,M2*> = len M1 + len M2
proof
  thus Sum Len <*M1,M2*> = Sum ((Len <*M1*>)^(Len<*M2*>)) by Th14
    .= Sum Len <*M1*>+Sum Len<*M2*> by RVSUM_1:75
    .= len M1+Sum Len <*M2*> by Lm4
    .= len M1+len M2 by Lm4;
end;
