theorem Th92:
  (f1 is_differentiable_in t0 & f2 is_differentiable_in t0 &
  f3 is_differentiable_in t0) & (g1 is_differentiable_in f1.t0 &
  g2 is_differentiable_in f2.t0 & g3 is_differentiable_in f3.t0)
  implies VFuncdiff(g1*f1,g2*f2,g3*f3,t0) =
  |[ diff(g1,f1.t0)*diff(f1,t0),diff(g2,f2.t0)*diff(f2,t0),
  diff(g3,f3.t0)*diff(f3,t0) ]|
proof
    assume that
A1: f1 is_differentiable_in t0 & f2 is_differentiable_in t0 &
    f3 is_differentiable_in t0 and
A2: g1 is_differentiable_in f1.t0 & g2 is_differentiable_in f2.t0 &
    g3 is_differentiable_in f3.t0;
    VFuncdiff(g1*f1,g2*f2,g3*f3,t0)
    = |[ diff(g1,f1.t0)*diff(f1,t0),diff(g2*f2,t0),diff(g3*f3,t0) ]|
      by A1,A2,FDIFF_2:13
   .= |[ diff(g1,f1.t0)*diff(f1,t0),diff(g2,f2.t0)*diff(f2,t0),
      diff(g3*f3,t0) ]| by A1,A2,FDIFF_2:13
   .= |[ diff(g1,f1.t0)*diff(f1,t0),diff(g2,f2.t0)*diff(f2,t0),
      diff(g3,f3.t0)*diff(f3,t0) ]| by A1,A2,FDIFF_2:13;
    hence thesis;
end;
