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*Proposition:*
For proving with knowledge `K` union {**exists** `x`: `F`}
the goal `G`, it suffices to prove
`G` with additional knowledge `F`[`x` <- `a`] for some object constant `a` that does not appear in
`K`, `G`, or `F`:

~~>

Kunion {existsx:F}G(

Kunion {existsx:F,F[x<-a]}Ganot inK,G,F)

"We prove `G`. Since we know (**exists** `x`: `F`), we
have have some `a` with `F`[`x` <- `a`].
Thus ...(proof of `G` with new knowledge `F`[`x`
<- `a`])."

*A formula ( exists x: F) in the knowledge base
is an "engine" which returns a new constant a about which we know
(only) F[x <- a].*

Author: Wolfgang Schreiner

Last Modification: November 30, 1999