+/*
+
+ This is a lookahead list for grace notes.
+
+ { ... X \grace Y Z ... }
+
+ normally, the ending of X is the start of Z. In case of a grace
+ note, we take off a little at the end of X. What is stored: START
+ (start point of X), LENGTH (length of X), GRACE_START (start_music
+ of Y), and the next fixup element.
+
+ This is also done for nested musics, i.e.
+
+ voiceA = \notes { \grace b16 c'2 }
+ voiceB = \notes { c'2 \voiceA }
+
+ the iterator for voiceB will contain a fixup entry with (START=0/1,
+ LENGTH=2/1, GRACE_START=(0G-1/16) )
+
+ Graces at the start of a sequential music iterator are handled
+ by initting here_mom_ with Music::start_music (); no fixups are needed.
+
+*/
+struct Grace_fixup