reserve a,b for Real,
  i,j,n for Nat,
  M,M1,M2,M3,M4 for Matrix of n, REAL;

theorem
  M1-M2 is_less_or_equal_with M3 implies M1-M3 is_less_or_equal_with M2
proof
  assume
A1: M1-M2 is_less_or_equal_with M3;
A2: Indices M1 = [:Seg n, Seg n:] by MATRIX_0:24;
A3: len M1=len M3 & width M1=width M3 by Lm3;
A4: Indices (M1-M2) = [:Seg n, Seg n:] by MATRIX_0:24;
A5: Indices (M1-M3) = [:Seg n, Seg n:] by MATRIX_0:24;
A6: len M1=len M2 & width M1=width M2 by Lm3;
  for i,j st [i,j] in Indices (M1-M3) holds (M1-M3)*(i,j)<=M2*(i,j)
  proof
    let i,j;
    assume
A7: [i,j] in Indices (M1-M3);
    then (M1-M2)*(i,j)<=M3*(i,j) by A1,A4,A5;
    then M1*(i,j)-M2*(i,j)<=M3*(i,j) by A2,A5,A6,A7,Th3;
    then M1*(i,j)-M3*(i,j)<=M2*(i,j) by XREAL_1:12;
    hence thesis by A2,A5,A3,A7,Th3;
  end;
  hence thesis;
end;
