* lily/context-property.cc (execute_general_pushpop_property):

* lily/context-property.cc (execute_general_pushpop_property):
robustness checks.

* lily/parser.yy (music_property_def): allow \override #'a #'b =
#c too.

* lily/context-property.cc (lookup_nested_property): new function.
(evict_from_alist): new function.
(general_pushpop_property): new function.
(execute_general_pushpop_property): rewrite. Support nested
properties too.