theorem Th94:
  (f1 is_differentiable_in t0 & f2 is_differentiable_in t0 &
  f3 is_differentiable_in t0) & (f1.t0 <> 0 & f2.t0 <> 0 & f3.t0 <> 0)
  implies VFuncdiff(f1^,f2^,f3^,t0) =
  - |[ diff(f1,t0)/(f1.t0)^2,diff(f2,t0)/(f2.t0)^2,
  diff(f3,t0)/(f3.t0)^2 ]|
proof
    assume that
A1: f1 is_differentiable_in t0 & f2 is_differentiable_in t0 &
    f3 is_differentiable_in t0 and
A2: f1.t0 <> 0 & f2.t0 <> 0 & f3.t0 <> 0;
    set p = |[ diff(f1,t0)/(f1.t0)^2,diff(f2,t0)/(f2.t0)^2,
    diff(f3,t0)/(f3.t0)^2 ]|;
    VFuncdiff(f1^,f2^,f3^,t0)
    = |[ -diff(f1,t0)/(f1.t0)^2,diff(f2^,t0),diff(f3^,t0) ]|
      by A1,A2,FDIFF_2:15
   .= |[ -diff(f1,t0)/(f1.t0)^2,-diff(f2,t0)/(f2.t0)^2,diff(f3^,t0) ]|
      by A1,A2,FDIFF_2:15
   .= |[ -diff(f1,t0)/(f1.t0)^2,-diff(f2,t0)/(f2.t0)^2,
      -diff(f3,t0)/(f3.t0)^2 ]| by A1,A2,FDIFF_2:15
   .= |[ (-1)*p.1,(-1)*p.2,(-1)*p.3 ]|
   .= - |[ diff(f1,t0)/(f1.t0)^2,diff(f2,t0)/(f2.t0)^2,
      diff(f3,t0)/(f3.t0)^2 ]| by Lm1;
    hence thesis;
end;
