+ (completionFactor ,rational-or-procedure?
+"When @code{Completion_heads_engraver} and
+@code{Completion_rest_engraver} need to split a note or rest with a
+scaled duration, such as @code{c2*3}, this specifies the scale factor
+to use for the newly-split notes and rests created by the engraver.
+
+If @code{#f}, the completion engraver uses the scale-factor of
+each duration being split.
+
+If set to a callback procedure, that procedure is called with the
+context of the completion engraver, and the duration to be split.")