reserve h,r,r1,r2,x0,x1,x2,x3,x4,x5,x,a,b,c,k for Real,
  f,f1,f2 for Function of REAL,REAL;

theorem
  f is constant implies [!f,x0,x1,x2!] = 0
proof
  assume
A1: f is constant;
  then [!f,x0,x1,x2!] = (0 qua Nat-[!f,x1,x2!])/(x0-x2) by DIFF_1:30
    .= (0 qua Nat-(0 qua Nat))/(x0-x2) by A1,DIFF_1:30;
  hence thesis;
end;
