.contains(edge)) {
if (edge.getProperty(FCOLOR_KEY) == null) {
edge.setProperty(FCOLOR_KEY, FRED);
if (element.equals(edge.getTarget())) {
edge.reverse();
}
} else if (edge.getIntProperty(FCOLOR_KEY) == FRED.intValue()) {
edge.setProperty(FCOLOR_KEY, FBLACK);
} else if (edge.getIntProperty(FCOLOR_KEY) == FBLACK.intValue()) {
throw new IllegalArgumentException("cannot meet black vertex");