problemsOf method
The problems of the selection with apps, the apps of the matrix: a
name of only that none of them has, and those of everyModuleApps
with the apps with every module among them.
Implementation
List<String> problemsOf(List<MatrixApp> apps) => [
for (final name in only ?? const <String>{})
if (!apps.any((app) => app.name == name))
'No app of the matrix is $name.',
...everyModuleApps.problemsOf([
for (final app in apps)
if (app.everyModuleWith != null) app,
]),
];