`completion-pcm--merge-completions', is assumed to match every
string in COMPLETIONS.
-If `completion-lazy-hilit' is nil, return a deep copy of
-COMPLETIONS where each string is propertized with
-`completion-score', a number between 0 and 1, and with faces
-`completions-common-part', `completions-first-difference' in the
-relevant segments.
+If `completion-lazy-hilit' is nil, return a deep copy of COMPLETIONS
+where each string is propertized with faces `completions-common-part'
+and `completions-first-difference' in the relevant segments.
Else, if `completion-lazy-hilit' is t, return COMPLETIONS
unchanged, but setup a suitable `completion-lazy-hilit-fn' (which