North & Hillard Ex. 196: the following is to be translated into Latin: "He (Hannibal) had almost reached the top of the Alps, when some old men came to him in the guise of envoys. The misfortunes of others, they said, had been a warning to them, and they preferred to make trial of the friendship rather than the might of the Carthaginians, and were ready to do whatever he wished. Hannibal, considering that he must not rashly either trust or slight them, accepted them as guides,"
The tricky part (Answer Book): "...quos Hannibal duces quidem accepit (accepted those who are guides, the English requires "as guides"-ducendos?), neque iis a se temere credendum ratus neque omnino asperandos eos esse;".
The narrator is reporting what the fake envoys said to Hannibal—indirect speech requiring the acc.-infin. construction for the indirect statement: "omnino aspernandos eos esse"—they had not to be offended at all. The preceding gerundive is given differently—neuter, impersonal "credendum"; not, "credendum est"—is this permissible, omitting "est"? Presumably, given that credo takes dative, dative-plural "iis" must be deployed (for them to be believed). In the acc.-inf. credendos eos esse—"eos" would clash with "iis".
Is this why the impersonal construction has been used?
In all the examples I have found on this, "est" has been deployed eg.
"mihi currendum est"—I must run (it ought to be run by me);
"Romanis cum hostibus pugnandum est"—the Romans ought to fight with the enemy (it ought to be fought, by the Romans, with the enemy).
Initially, thought that these were gerundives-of-obligation; but, that would have needed Hannibal to go into the dative, Hannibali, as the person upon whom the obligations were falling--neither for him (a se) to be believing nor offending.