In thinking about "supererogatory" it occurred to me that there ought to be a word "erogatory" meaning called for, obligatory. No can find.
Did find "suberogatory" defined by user as "blameworthy but not obligatory". I have trouble thinking what conduct would be both blameworth and obligatory. (The Devil made me do it)?
The same site had word new to me: "deontic logic". Couln[t find "deontic" but did find dandy word "deontology":
deontology
n.
5< Gr deon (gen. deontos), that which is binding, necessity < dein, to bind (see DIADEM) + 3LOGY6 the ethical doctrine which holds that the worth of an action is determined as by its conformity to some binding rule rather than by its consequences
de[on[to[log[i[cal 7dc 9n#t! l9j4i k!l8
adj.

and derogatory?

here's what I found in OED:

1) erogate [obs.] t.v. - To pay out, expend; to distribute.
No Man can Supererogate till he have first erogated.
thus, erogation - expenditure, esp. in the bestowal of gifts, almsgiving

2) (didn't find suberogatory, but) subtererogation, the performance of less than is required (opp. of supererogation)

3) pretererogation, nonce-wd. [after supererogation], performance beyond or outside of what is demanded or required.

it seems all of these words were pre-formed in Latin and came to us with prefixes attached, therefore erogatory would be, at best, an inkhorn term.

4) deontics [Philos.]

[f. Gr. dŒom, deoms- (see deontology) + -ic.]
A. n.

1. pl. [After ethics, eudemonics, etc.] (See quots.)
a1866 J. Grote Moral Ideals (1876) vii. 102 A science of duty (deontics or deontology). 1906 J. S. Stuart-Glennie in Sociol Pap. II. 250 The second order of ethical sciences.. form the contents of three classes of sciences—Economics, Deontics, and Juridics.
2. sing. (See quot.)
1926 Mind XXXV. 395 Ethical arguments+should be able to exhibit their ‘deontic’ in the same way as inference reveals its ‘logic’.

B. adj. Of or relating to duty, obligation, etc.

1. The word, erogate, appears to have the same root word as derogate/ interrogate/ abrogate/ arrogate, etc. Onelook confirms it to be the Latin, rogare, to ask. How did erogate come to acquire its association with money? And if derogate and erogate both trickled down with their Latin prefixes intact, why, despite sharing a common root and similar-meaning prefixes, are they two completely disconnected words in current usage?

2. Why is 'subter' the prefix and not, a simple, 'sub'? To my mind, 'subter' is a Latin prefix with a distinct connotation of, something unknown or secretive. Why is it used as the antonym for supererogatory?

Lovely words. wwh and tsuwm, thank you.