Completing <extend/> support, phase 5
Completing <extend/> support, phase 5

grame-cncm/libmusicxml,grame-cncm/libmusicxml,dfober/libmusicxml,grame-cncm/libmusicxml,grame-cncm/libmusicxml,arshiacont/libmusicxml,arshiacont/libmusicxml,grame-cncm/libmusicxml,grame-cncm/libmusicxml,arshiacont/libmusicxml,arshiacont/libmusicxml,dfober/libmusicxml,dfober/libmusicxml,arshiacont/libmusicxml,dfober/libmusicxml