Description: If as a conseqence of the occurrence of the circumstance ph , when the
obstacle ps is removed, ch takes place, then from the circumstance
that ch does not take place while ph occurs the occurrence of the
obstacle ps can be inferred. Closed form of con1d . Proposition 34
of Frege1879 p. 45. (Contributed by RP, 24-Dec-2019)(Proof modification is discouraged.)