createGroup(ident id, ident groupId, string day, string time, string location, integer maxSize) {
  assume exists //exercises/exercise[id];
  assume not exists //exercises/exercise[id]/groups/group[groupId];
 
  assume maxSize >= 0;
  assume day = "Monday" || day = "Tuesday" || ...;
 
  insert //exercises/exercise[id]/groups <group id=[groupId] day=[day] time=[time] location=[location] maxSize=[maxSize] />;
} # roles: assistant[id]