Consider the following dialogue:
Mark: ``Sell me a bottle of whisky"
Lucy: ``Are you over 18?"
A first way for analyzing Mark's question is by defining a complex discourse action where, given a request, different reactions are possible: accept the request, refuse it, possibly requesting him to check the truth value of the preconditions and constraints of the action (if they are unknown to Lucy), etc.
Our problem-solving analysis can explain the behavior in a more general way. From the interpretation of Mark's request, it is possible to infer that he wants that Lucy accepts the intention of selling him the bottle (in this way, the intention could become a Joint Intention and Lucy will execute the action). Lucy can decide to accept or to refuse the intention. If she accepts it, she must plan to give him the bottle (``Try-execute(L,M,Sell(L,M,Bottle))''); so, she must check the constraints of the domain action: in this case, whether Mark is over 18. The best way to satisfy the goal (``Knowif(L,over-18(M))'') is to ask him, so she does it. But suppose that Lucy replies ``Can you give me your identity card?''. In terms of problem solving library what has happened is that another plan has been chosen for satisfying ``Knowif(L,over-18(M))''. Here, however, the plan does not involve just asking for the required information (i.e. a linguistic action), but also a domain action (reading the identity card), which only indirectly requires the execution of a linguistic action (asking for the card).