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from "Dion," by Plutarch (translated by A.H. Clough)
Analysis Key through KISS Levels Four and Five

       {At this} their meeting [#1], the subject-matter {of their discourse}

{in general} was human virtue (PN), | but, more particularly, they

disputed {concerning [#2] fortitude}, [Adj. to "fortitude" which [#3] [Inj [#3]

Plato proved] tyrants, {of all men}, had the least pretense to [#3] ]; |

and thence proceeding to treat [#4] {of justice}, *Plato* asserted the happy

estate (DO) {of the just}, and the miserable condition (DO) {of the unjust};

arguments [#5] [Adj. to "arguments" which Dionysius would not hear

out], | but, feeling [#6] himself [#7], [Adv. to "feeling" as it were]

convicted [#7] {by his words}, and much displeased [#7] to see [#7] the

rest [#8] {of the auditors} full [#8] {of admiration} {for the speaker} and 

captivated [#8] {with his doctrine}, {at last}, exceedingly exasperated [#6]

he asked the philosopher (IO) {in a rage}, [DO what business (DO)

he had {in Sicily}]. | {To which} [#9] Plato answered, [DO "I came

to seek a virtuous man [#10]."] | "It seems then," [Inj replied Dionysius,]

[Adv. to "seems" "you have lost your labor (DO)."] |


Notes
1. I suspect that some people will consider "At this their meeting" as the full prepositional phrase, and I would accept that answer from any student. On the other hand, many readers will probably process (as I do) "At this" as the prepositional phrase, and "meeting" (modified by "their") as an appositive to "this."
2. "Concerning" is not included in the lists of prepositions either on this KISS site, or in any other list that I remember seeing, so I would not expect students working at Levels Two or Three to get this phrase, even though "concerning" here means "about." I expect that most students will simply ignore the phrase, such that it should not cause any problem for them. At Level Four, when students are dealing with gerunds and gerundives, "concerning" should come to students' attention because of its "-ing" form. At that point they may, as I did, try to explain it as a gerundive to "they." I wasn't happy with that explanation so I looked "concerning" up in my Webster's New Collegiate Dictionary (1961), where I found it listed as a preposition. 
3. It would be very interesting to see how traditional grammarians would explain a sentence such as this one, but traditional grammarians don't usually explain sentences. The complexity of the syntax here may become clearer if we reword the subordinate clause as a main clause -- "Plato proved that tyrants, of all men, had the least pretense to fortitude." With it written this way, traditional grammarians would explain "Plato proved" as the main subject and verb and the rest of the sentence as a subordinate clause that functions as a direct object of "proved." KISS, because it is based on a psycholinguistic model of how the brain decodes language, treats "Plato proved" as a subordinate clause that functions as an interjection
     Note also that when the main clause ("Plato proved that tyrants, of all men, had the least pretense to fortitude.") is subordinated as an adjective to "fortitude," "fortitude" is replaced by the pronoun "which" and moved to the beginning of the clause, where it functions simultaneously as the subordinating conjunction and the object of the preposition "to."
     Because of the initial ambiguity of the subject of "asserted," and because of the semicolon after "to," I have considered "to" as the end of a main clause and inserted an "ellipsed" "Plato" as the subject of "asserted."
4.  Close analysis leads to the conclusion that "proceeding" is a gerundive to the following ellipsed "Plato," since it was Plato who proceeded to treat of justice. In normal reading, however, the style here could be very ambiguous. As a verbal adjective, "proceeding" should chunk to the nearest noun that would make sense as the performer of the "proceeding." Normally, the noun can be either before or after the gerundive. In this case, there is no noun before "proceeding" and after the semicolon. Before the semicolon, working backward to find the noun closest to "proceeding," we first encounter "tyrants." Some readers will probably mentally test "tyrants" as the subject of "proceeding," but that reading does not end up making sense. Thus, readers need to look for another possible subject, and that leads to "Plato," the subject of "proved." I would suggest, however, that even "Plato" is not totally satisfactory as the subject of "proceeded," for, if we move back in the text still further, we find the "they" in "they disputed," and this "they" may also be the subject of "proceeding." The difference in meaning may be slight, but it may also be significant if we clear away the other ideas in the sentence. In one version, we would get "They disputed concerning fortitude, and thence they proceeded to treat of justice." In the other version, we get a much more controlling view of Plato -- "They disputed concerning fortitude, and thence Plato proceeded to treat of justice." (The infinitive "to treat" functions as the direct object of "proceeding.")
5.  Modern styles tend toward shorter, less complicated sentences, and semicolons are usually used only to separate main clauses. We could consider this the end of a main clause and then insert "these were" to make "arguments" a predicate noun in a new main clause, but to do so would obscure the complexity of Clough's style. Trained readers will probably not process "arguments" in this way; instead they will probably read it as an appositive to the previous assertions.
     It is an interesting and complex appositive in that it depends on assumptions. Superficially, we can say that "arguments" is an appositive to "the happy estate of the just, and the miserable condition of the unjust." The latter, however, are conclusions, not arguments. We are told that Plato asserted these ideas, but we are not given his arguments. Even careful readers who are unfamiliar with Plato (and his tight logical arguments) might find this appositive very confusing, especially if they are, as many of my college students are, unfamiliar with appositives in the first place.
6.  "Feeling" is a gerundive. It modifies the "he" several lines further into the text. Unlike the problem with "proceeding," however, in this case there is no doubt about who is "feeling." The "he" means "Dionysius," and "Dionysius" is also clearly the last noun before "feeling" in the text. 
7.  "Himself" is the subject and "convicted" and "displeased" are gerundives that function as predicate adjectives to an ellipsed infinitive "*to be*" The infinitive phrase functions as the direct object of "feeling." [Traditional grammarians and many modern linguists would probably discuss "convicted" and "displeased" as objective or subjective complements.] The infinitive "to see" functions as an adverb to "displeased." (Alternatively, some people may justifiably want to explain "himself  ... convicted" and "displeased" as the core of a noun absolute that functions as the direct object of "feeling.")
8. There are at least two ways of looking at "the rest ... full ... and captivated ..." within the KISS Approach. The first is to consider "rest" as the direct object of "to see" and then to explain "full" and "captivated" as post-positioned adjectives, i.e., reductions of subordinate clauses -- to see the rest of the auditors *who were* full of admiration for the speaker and *who were* captivated with his doctrine. The other option is to assume ellipsed infinitives --  to see the rest of the auditors *be* full of admiration for the speaker and *be* captivated with his doctrine. 
9. Most modern editors would probably consider "which" as a subordinating conjunction here, and therefore change this to "To this ...."
10. "Man" is the direct object of the infinitive "to seek" which functions as an adverb (of purpose) to "came."