Bolzano on Obligation and Supererogation

Here is a curious passage from Bernard Bolzano's Wissenschaftslehre, sec. 147 (HT: V.V.):

. . . I take the concept of obligation in such a wide sense that it holds of every resolution which can be termed morally good, whether it is a definite duty or  or merely meritorious, so that we can say of both kinds that they ought to be performed. Thus I say, for example, that one ought not lie, which is a duty; and I also say that we ought to be charitable, which is not a duty, but merely meritorious . . . (Rolf George tr., p. 192)

I see it differently. The obligatory does not include the meritorious or supererogatory.  Both pertain to the actions and resolutions of rational beings.  The difference is that supererogatory actions are not required, whether morally or legally, whereas obligatory actions are.  

The obligatory is what one MUST do. The obligatory is the sphere of moral necessity.

The impermissible is what one MUST NOT do. The impermissible is the sphere of moral impossibility.

The permissible is what one MAY do. The permissible is the sphere of moral contingency.

The supererogatory is a proper subset of the permissible. Its intersection with the obligatory is null, and its intersection with the impermissible is also null.