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(** *   Value of the derivative of the reciprocal function        *)
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Lemma derive_pt_recip_interv_prelim0 (f g:R->R) (lb ub x:R)
(Prf:derivable_pt f (g x)) (Prg:derivable_pt g x)
: lb < ub ->
lb < x < ub ->
(forall x, lb < x < ub -> (comp f g) x = id x) ->
derive_pt f (g x) Prf <> 0 ->
derive_pt g x Prg = 1 / (derive_pt f (g x) Prf).