Model:
    locations+=Coordinates*
;
Coordinates:
    '(' x=INT ',' y=INT ')'
;

@Check
def checkLocation(Model model) {
    val locationSet = newHashSet
    for (location : model.locations) {
        if (!locationSet.add(location)) {
            error('Locations must be unique.', location, LOCATION)
        }
    }
}