theorem Th5:
  cD(f,h)/.x = f/.(x+(2*1.F)"*h) - f/.(x-(2*1.F)"*h)
proof
  dom (Shift(f,(2*1.F)"*h) - Shift(f,-(2*1.F)"*h))
  = the carrier of V by FUNCT_2:def 1;
  hence cD(f,h)/.x = Shift(f,(2*1.F)"*h)/.x - Shift(f,-(2*1.F)"*h)/.x
  by VFUNCT_1:def 2
  .= f/.(x+(2*1.F)"*h) - Shift(f,-(2*1.F)"*h)/.x by Def2
  .= f/.(x+(2*1.F)"*h) - f/.(x-(2*1.F)"*h) by Def2;
end;
