Keep a staff alive with multiple layers

This allows the `VerticalAxisGroup.remove-layer'
property to accept a list of values. The layer
will stay alive with any other member of the
Keep_alive_together_engrave group with a
remove-layer value in that list.

The principal reason for this patch was to
allow the use of MarkLine contexts in a Frenched
score, where the context should stay alive with
any single staff in a StaffGroup.

This implementation should also allow additional
flexibility with ossia and divisi staves.

2 files changed, 39 insertions, 4 deletions

diff --git a/lily/hara-kiri-group-spanner.cc b/lily/hara-kiri-group-spanner.cc
index 06410786a6..76207a5c92 100644
--- a/lily/hara-kiri-group-spanner.cc
+++ b/lily/hara-kiri-group-spanner.cc
@@ -190,9 +190,11 @@ Hara_kiri_group_spanner::add_interesting_item (Grob *me, Grob *n)
 ADD_INTERFACE (Hara_kiri_group_spanner,
                "A group spanner that keeps track of interesting items.  If it"
                " doesn't contain any after line breaking, it removes itself"
-               " and all its children.  Children may be prioritized in layers"
-               " via @code{remove-layer}, in which case only the"
-               " lowest-numbered non-empty layer is retained.",
+               " and all its children.  Greater control can be exercised via"
+               " @code{remove-layer} which can prioritize layers so only the"
+               " lowest-numbered non-empty layer is retained; make the layer"
+               " independent of the group; or make it dependent on any other"
+               " member of the group",
 
                /* properties */
                "items-worth-living "
diff --git a/lily/keep-alive-together-engraver.cc b/lily/keep-alive-together-engraver.cc
index 9b1cbe4a23..8931a819c6 100644
--- a/lily/keep-alive-together-engraver.cc
+++ b/lily/keep-alive-together-engraver.cc
@@ -22,6 +22,7 @@
 #include "engraver.hh"
 #include "grob.hh"
 #include "grob-array.hh"
+#include "international.hh"
 
 #include "translator.icc"
 
@@ -64,12 +65,44 @@ Keep_alive_together_engraver::finalize ()
         {
           if (i == j)
             continue;
+
+          if (scm_is_symbol (this_layer))
+            {
+              if (scm_is_eq (this_layer, ly_symbol2scm ("any")))
+                {
+                  // layer is kept alive by any other layer
+                  live->add (group_spanners_[j]);
+                  continue;
+                }
+              else if (scm_is_eq (this_layer, ly_symbol2scm ("above")))
+                {
+                  // layer is kept alive by the layer preceding it
+                  if (i == j + 1)
+                    live->add (group_spanners_[j]);
+                  continue;
+                }
+              else if (scm_is_eq (this_layer, ly_symbol2scm ("below")))
+                {
+                  // layer is kept alive by the layer following it
+                  if (i == j - 1)
+                    live->add (group_spanners_[j]);
+                  continue;
+                }
+              else
+                {
+                  group_spanners_[i]->warning (_f ("unknown remove-layer value `%s'",
+                                                   ly_symbol2string (this_layer).c_str ()));
+                  continue;
+                }
+            }
+
           SCM that_layer = group_spanners_[j]->get_property ("remove-layer");
+
           if (scm_is_false (that_layer))
             continue;
           if (!scm_is_integer (this_layer))
             {
-              // Unspecified layers are kept alive by anything else
+              // unset layers are kept alive by all but ignored layers
               live->add (group_spanners_[j]);
               continue;
             }