reserve x for set,
  D for non empty set,
  k,n,m,i,j,l for Nat,
  K for Field;

theorem
  for A,B,C being Matrix of REAL st len A=len B & width A=width B & len
  C=width A & len A>0 & len C>0 holds (A-B)*C=A*C - B*C by Th16;
