EWD249 - 29

The main point is that the values of these indices are outside the programmer's control; they are defined (either by the write-up of his program or by the dynamic evolution of the current computation) whether he likes it or not. They provide independent co-ordinates in which to describe the progress of the computation, a "variable-independent" frame of reference in which meanings to variables can be assigned.

There is, of course, even with the free use of jumps, a programmer independent co-ordinate system in terms of which the progress of a sequential computation can be described uniquely, viz. a kind of normalized clock that counts the number of "discrete points of computation progress" passed since program start. It is unique, but utterly unhelpful, because the textual index is no longer a constituent component of such a co-ordinate system.

The moral of the story is that when we acknowledge our duty to control the computations (intellectually!) via the program text evoking them, that then we should restrict ourselves in all humility to the most systematic sequencing mechanisms, ensuring that "progress through the computation" is mapped on "progress through the text" in the most straightforward manner.