Two ordinary moments from a working week. A function ships clean,
and months later fails on a case its type checker looked straight
at and passed. A service that shipped clean a year ago, untouched
since, will no longer run - nothing in it moved; everything under
it did. The first says a rule can never cover every case it governs;
the second, that an artefact can never be finished, only kept in
being.

This is a book about those two refusals and its single claim of
them: neither ever closes. One opening is logical, a limit no
procedure escapes, sharpened here into a theorem; the other is
temporal, the upkeep of software having the form of labor -
consumed in the doing, setting nothing down for good. Held apart as
two lenses, they are shown to touch at one place a programmer can
point to: the commit in which a custom kept by hand hardens into a
rule a machine enforces. And the enclosure settles nothing: it moves
the unruled margin rather than removing it, opening a fresh frontier at
the new rule's edge. From that join follows an account of what a
maker does where nothing comes to rest - not finishing, which no
one can do, but staffing the unfinishable in the open. The fifth
volume in the Code Crafting series, written to be read alone.