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 &
  width C=len A & len C>0 & len A>0 holds C*(A-B)=C*A -C*B by Th20;
