 Intuitively, a sequential function is one whose evaluation proceeds serially: this means that it is possible to schedule the evaluation of its arguments so that the evaluation of the function terminates with the correct value; if the evaluation of one of them diverges, the whole evaluation diverges. At each stage of this process there is an argument whose value is needed to obtain more information on the output of the function. In order to account for this causal structure of computations at the semantical level, we need to enrich the domain structure so that the order on the elements reflect the happening of computational events and their causal order. This suggests another way of interpreting the abstract notion of information that motivated the axioms of a cpo in  Section 2.3.1.  Now, 