In this section we give the properties of the functions and , which are central for representing question/answer discourses. Since the and functions are treated in the style of situation calculus, we do not need to change our basic theory of context, but simply give the axioms that formalize the two functions.

Intuitively, the function
will set up a context in which the reply to the
question will be interpreted.
For example, the context resulting in asking some proposition *p*
will have the property that in that context
will be interpreted as *p*.
Thus only changes the semantic state of the discourse context.
The function will do a simple update of information:
the formulas true in
the context resulting in replying *p* in will be exactly those
formulas which are conditionally true on *p* in .
Thus the function only changes the epistemic state of the discourse
context.
We now make these notions more precise.

The following axioms characterize the functions and .

**interpretation axiom (propositional)**-
if is a closed formula, then
**frame axiom (propositional)**-
if is a closed formula, and does not
occur in , then
**interpretation axiom (qualitative)**-
if
*x*is the only variable occurring free in , then **frame axiom (qualitative)**-
if
*x*is the only variable occurring free in , and does not occur in , then **reply axiom**-

We proceed to illustrate the axioms and their use with an example.

Sun Jul 12 14:45:30 PDT 1998