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

theorem
  M1 is_less_or_equal_with M2-M3 implies M3 is_less_or_equal_with M2-M1
proof
  assume
A1: M1 is_less_or_equal_with M2-M3;
A2: Indices M1 = [:Seg n, Seg n:] by MATRIX_0:24;
A3: Indices M3 = [:Seg n, Seg n:] by MATRIX_0:24;
A4: len M2=len M3 & width M2=width M3 by Lm3;
A5: Indices M2 = [: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 M3 holds M3*(i,j)<=(M2-M1)*(i,j)
  proof
    let i,j;
    assume
A7: [i,j] in Indices M3;
    then M1*(i,j)<=(M2-M3)*(i,j) by A1,A2,A3;
    then M1*(i,j)<=M2*(i,j)-M3*(i,j) by A5,A3,A4,A7,Th3;
    then M3*(i,j)<=M2*(i,j)-M1*(i,j) by XREAL_1:11;
    hence thesis by A5,A3,A6,A7,Th3;
  end;
  hence thesis;
end;
