reserve n,m,k,i for Nat,
  h,r,r1,r2,x0,x1,x2,x for Real,
  S for Functional_Sequence of REAL,REAL,
  y for set;
reserve f,f1,f2 for Function of REAL,REAL;

theorem
  [!f,x0,x1!] = [!f,x1,x0!]
proof
  thus [!f,x0,x1!] = (-(f.x1-f.x0))/(-(x1-x0)) .= [!f,x1,x0!] by XCMPLX_1:191;
end;
