@savask thanks a lot for your detailed explanation how reverse engineering 
can be performed with mmj2. Following your "tutorial", I was able to create 
my first theorem (from No. 6 of your list):

h1::Test.1        |- ( ph -> X e. A )
h2::Test.2        |- ( ph -> th )
h3::Test.3        |- ( ph -> ta )
h4::Test.4        |- ( x = X -> ( ps <-> th ) )
h5::Test.5        |- ( x = X -> ( ch <-> ta ) )
d5:4,5:anbi12d   |- ( x = X -> ( ( ps /\ ch ) <-> ( th /\ ta ) ) )
d4:d5:rspcev       |- ( ( X e. A /\ ( th /\ ta ) ) -> E. x e. A ( ps /\ ch 
) )
qed:1,2,3,d4:syl12anc 
                   |- ( ph -> E. x e. A ( ps /\ ch ) )

with $d A x $. $d X x $.  $d ta x $. $d th x $.

But is this a useful theorem? Although it looks inconspicuous, I have run 
the minimize script looking for proofs using ~rspcev (there are more than 
5000 of them), and stopped the script after more than 100 proofs were found 
which could be minimized by using the new theorem. This shows that there is 
a high potential that such theorems can be useful for a lot of 
minimizations. 

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