theorem
  f1 is_differentiable_in t0 & f2 is_differentiable_in t0 &
  f3 is_differentiable_in t0 implies VFuncdiff(f1,f2,f3,t0) =
  diff(f1,t0)*<e1>+diff(f2,t0)*<e2>+diff(f3,t0)*<e3> by Th31;
