code: private boolean continuousColocation(ReconfigurationProblem rp, List<Slice> dSlices) {
  
        Set<VM> alreadyRunning = new HashSet<>();
          Mapping map = rp.getSourceModel().getMapping();
          Node loc = null;
          for (VM vm : cstr.getInvolvedVMs()) {
              if (map.isRunning(vm)) {
                alreadyRunning.add(vm);
                  Node node = map.getVMLocation(vm);
                  if (loc == null) {
                      loc = node;
                  } else if (!loc.equals(node)) {
                      rp.getLogger().error("Some VMs in '{}' are already running but not co-located", cstr.getInvolvedVMs());
                      return false;
                  }
              }
          }
          if (loc != null) {
            if (cstr.getInvolvedVMs().size() == 1 && dSlices.size() == 1) {
              // The VM may migrate, we don't care as it is alone.
              return true;
            }
              return placeDSlices(rp, dSlices, rp.getNode(loc));
          }
          return true;
      }
docs: /*
  Check for the already running VMs and force co-location if any.