<retrace> contains a sequence of writing which has been retraced, for example by over-inking, to clarify or fix it. [11.3.4.3 Fixation and Clarification]
Multiple retraces are indicated by nesting one retrace within another. In principle, a retrace differs from a substitution in that second and subsequent rewrites do not materially alter the content of an element. Where minor changes have been made during the retracing action however these may be marked up using del, add, etc. with an appropriate value for the change attribute.