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 Th20:
  Sum Width <*M1,M2*>=width M1+width M2
proof
  thus Sum Width <*M1,M2*> = Sum ((Width <*M1*>)^(Width<*M2*>)) by Th18
    .= Sum Width <*M1*>+Sum Width<*M2*> by RVSUM_1:75
    .= width M1+Sum Width <*M2*> by Lm5
    .= width M1+width M2 by Lm5;
end;
