Usage: musicXmlDiff INPUT_FILE COMPARE_FILE
Compares two MusicXML files.
Diff output is to stdout.
The two MusicXML files are first standardized using xmllint before the diff is performed.
Unlike Unix diff, all '<' lines appear before all '>' lines.
INPUT_FILE: the MusicXML input filename.
COMPARE_FILE: the filename of the MusicXML file that the INPUT_FILE is compared to.