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This ended up a lot longer than I expected, but there's always a lot of machinery involved in extensible structures, and in this case the definition (unaltered from set.mm) allows either an extensible structure or an ordered pair of vertexes and edges.

Most of the discouraged theorems are also discouraged in set.mm, although I did re-intuitionize 2strstr to reflect changes in set.mm since the first one, and that accounts for a few of the entries.

One of the biggest differences from set.mm is that 2o ~<_ A is well behaved, in iset.mm, for saying a set has at least two elements (https://us.metamath.org/mpeuni/rex2dom.html is provable and is added here) whereas 2 <_ ( # ` A ) is not (at least with current theorems, # works on finite sets or on infinite sets, but not on arbitrary sets).

This is the syntax and df-edgf .  Copied without change from set.mm.
Stated as in set.mm.  The proof is the set.mm proof with small changes
to use strnfvnd
This is the syntaxes , df-vtx , and df-iedg .  Copied without change
from set.mm.
This is vtxval from set.mm with a set existence condition added.  The
proof is taken from a portion of the set.mm proof with changes for set
existence.
This is iedgval from set.mm with a set existence condition added.  The
proof is taken from a portion of the set.mm proof, modified for
differences in set existence.
Stated as in set.mm.  The proof is based on the set.mm proof but needs
to be significantly longer for differences in set existence theorems.
Stated as in set.mm.  The proof is the set.mm proof with a very small
change for set existence.
Stated as in set.mm.  The proof is the set.mm proof with a small change
for differences in set existence.
Although this is similar to various existing theorems, we don't seem to
have quite this form until now.
Stated as in set.mm.  The proof is based on the iset.mm proof of en1 .
Stated as in set.mm.  The proof is the set.mm proof with some small
changes for differences in set existence theorems.
This is edgfiedgval from set.mm with a change to how we say a set has at
least two elements.  The proof is the set.mm proof with small changes.
This is funvtxval and funiedgval from set.mm with set existence
conditions added.  The proofs are the set.mm proofs with small changes.
This is implied by a comment, so say it explicitly.
This is hashdmpropge2 from set.mm with a change to how we specify that a
set has at least two elements.  The proof is the set.mm proof with some
adjustments to use rex2dom in place of hashge2el2difr .
This is structvtxvallem from set.mm but changes the way we say that a
set has at least two elements.  The proof is the set.mm proof with a
number of small changes.
This is like 2strstrg in iset.mm but adapted to current conventions
around extensible structure indexes.  It is the same as 2strstr in
set.mm but with set existence conditions added.  The proof is slightly
modified from the 2strstrg proof.

Mark 2strstrg as discouraged because it hardcodes the index one.
Stated as in set.mm.  The proof is the set.mm proof with some
adjustments (including some set existence ones which make the proof
longer).
Stated as in set.mm.  The proof is the set.mm proof with small
changes in various places.
This is structgrssvtxlem from set.mm with changes to how we specify
that a set has at least two elements.  The proof is the set.mm
proof with several small changes.
Stated as in set.mm.  The proof is the set.mm proof with small changes.
Stated as in set.mm.  The proof is the set.mm proof with several small
changes.
This is struct2grstr from set.mm with set existence conditions added.
The proof is the set.mm proof with small changes.
Stated as in set.mm.  The proof is the set.mm proof with small
changes.
Stated as in set.mm.  The proof is the set.mm proof with one small
change.
This is grstructd from set.mm with a change to how we specify that a set
has at least two elements.  The proof is the set.mm proof with small
changes.
This is grstructeld from set.mm with changes to how we specify that a
set has at least two elements.  The proof is the set.mm proof with small
changes.
Update text for basprssdmsets
( vf wcel w3a wf1o wa cen wbr cv wex f1oeq1 spcegv imp 3ad2antl1 wb breng
3adant1 adantr mpbird ) CDHZAEHZBFHZIZABCJZKABLMZABGNZJZGOZUEUFUIUMUGUEUI
UMULUIGCDABUKCPQRSUHUJUMTZUIUFUGUNUEABGEFUAUBUCUD $.
$( $j usage 'f1oen4g' avoids 'ax-un'; $)
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'ax-rep' and 'ax-pow' shold be added (as mentioned in the comment).

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Hmm, interesting. Totally agree that the comment should match the $j usage. Looks like we can add 'ax-coll' for both f1oen4g f1dom4g. But ax-pow is used in f1oen4g (via breng) and f1dom4g (via brdom2g) so I'll adjust the comment.

en2prd.4 $e |- ( ph -> D e. Y ) $.
en2prd.5 $e |- ( ph -> A =/= B ) $.
en2prd.6 $e |- ( ph -> C =/= D ) $.
$( Two unordered pairs are equinumerous. (Contributed by BTernaryTau,
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"Two proper unordered pairs ..."


${
$d A f x y $.
$( A set equinumerous to ordinal 2 is a pair. (Contributed by Mario
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"... is an unordered pair."

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Reviewed up to "GRAPH THEORY". Only minor remarks. I will review section about graph theory later.

structvtxvallem.g $e |- G = { <. ( Base ` ndx ) , V >. , <. S , E >. } $.
$( Lemma for ~ structvtxval and ~ structiedg0val . (Contributed by AV,
23-Sep-2020.) (Revised by AV, 12-Nov-2021.) $)
structvtxvallem2dom $p |- ( ( V e. X /\ E e. Y ) -> 2o ~<_ dom G ) $=
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I think this should still be called "structvtxvallem", because it is still a lemma for ~ structvtxval and ~ structiedg0val only.

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Alternatively, this lemma could become a stand alone theorem, maybe called ~struct2slots2dom

<. ( .ef ` ndx ) , E >. } C_ G ) $.
$( Lemma for ~ structgrssvtx and ~ structgrssiedg . (Contributed by AV,
14-Oct-2020.) (Proof shortened by AV, 12-Nov-2021.) $)
structgrssvtxlem2dom $p |- ( ph -> 2o ~<_ dom G ) $=
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See comment for ~structvtxvallem2dom.

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Graph Theory reviewed, still only minor remarks about wording

It's fantastic to see how graph theory becomes intuitonized. Hopefully, the definition of graphs either as ordered pairs or as extensible structures was the most difficult part, and the rest can be done more easily.

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