@Override
public final boolean equals(Object o) {
if (o == this) return true;
if (!(o instanceof Occurrence)) return false;
Occurrence t = (Occurrence) o;
if (source != t.source) return false;
if (pos != t.pos) return false;
if (symbol == t.symbol) return true;
else {
if (symbol == null) return false;
return symbol.equals(t);
}
}
