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 implies -M2 is_less_or_equal_with -M1
proof
A1: Indices M1=[:Seg n, Seg n:] by MATRIX_0:24;
A2: Indices M2=[:Seg n, Seg n:] by MATRIX_0:24;
A3: Indices (-M2)=[:Seg n, Seg n:] by MATRIX_0:24;
  assume
A4: M1 is_less_or_equal_with M2;
  for i,j st [i,j] in Indices (-M2) holds (-M2)*(i,j)<=(-M1)*(i,j)
  proof
    let i,j;
    assume
A5: [i,j] in Indices (-M2);
    then M1*(i,j)<=M2*(i,j) by A4,A1,A3;
    then -M2*(i,j)<=-M1*(i,j) by XREAL_1:24;
    then (-M2)*(i,j)<=-M1*(i,j) by A2,A3,A5,Th2;
    hence thesis by A1,A3,A5,Th2;
  end;
  hence thesis;
end;
