Although some books demonstrate that the current implementation of Xt happens
to call callback procedures in the order in which they are registered, the
specification does not guarantee such a sequence, and supplemental
authoritative documents (i.e. the Asente/Swick volume) do say that the order
is undefined.  Because the callback list can be manipulated by both the
widget and the application, Xt cannot guarantee the order of execution.

In general, the callback procedures should be thought of as operating
independently of one another and should not depend on side-effects of other
callbacks operating; if a seqence is needed, then the single callback to be
registered can explicitly call other functions necessary.

[4/92; thanks to converse@x.org]