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

theorem Th28:
  for A,B,C being Matrix of n,REAL holds A*B*C=A*(B*C)
proof
  let A,B,C be Matrix of n,REAL;
A1: width B=n & len C=n by MATRIX_0:24;
  width A=n & len B=n by MATRIX_0:24;
  hence thesis by A1,MATRIX_3:33;
end;
