Added first mapper

src/main/java/beamline/mappers/DirectlyFollowsRelation.java

+package beamline.mappers;
+
+import org.deckfour.xes.model.XEvent;
+
+public class DirectlyFollowsRelation {
+
+	private String caseId;
+	public XEvent first;
+	public XEvent second;
+	
+	public DirectlyFollowsRelation(String caseId, XEvent first, XEvent second) {
+		this.caseId = caseId;
+		this.first = first;
+		this.second = second;
+	}
+	
+	public String getCaseId() {
+		return caseId;
+	}
+
+	public XEvent getFirst() {
+		return first;
+	}
+
+	public XEvent getSecond() {
+		return second;
+	}
+}

src/main/java/beamline/mappers/InfiniteSizeDirectlyFollowsMapper.java

+package beamline.mappers;
+
+import java.util.HashMap;
+import java.util.Map;
+
+import org.deckfour.xes.extension.std.XConceptExtension;
+import org.deckfour.xes.model.XEvent;
+import org.deckfour.xes.model.XTrace;
+
+import io.reactivex.rxjava3.annotations.NonNull;
+import io.reactivex.rxjava3.core.Observable;
+import io.reactivex.rxjava3.core.ObservableSource;
+import io.reactivex.rxjava3.functions.Function;
+
+public class InfiniteSizeDirectlyFollowsMapper implements Function<XTrace, ObservableSource<DirectlyFollowsRelation>> {
+
+	private Map<String, XEvent> map = new HashMap<String, XEvent>();
+	
+	@Override
+	public @NonNull ObservableSource<DirectlyFollowsRelation> apply(@NonNull XTrace t) throws Throwable {
+		String caseId = XConceptExtension.instance().extractName(t);
+		DirectlyFollowsRelation toRet = null;
+		
+		if (map.containsKey(caseId)) {
+			toRet = new DirectlyFollowsRelation(caseId, map.get(caseId), t.get(0));
+		}
+		
+		map.put(caseId, t.get(0));
+		
+		if (toRet == null) {
+			return Observable.empty();
+		} else {
+			return Observable.just(toRet);
+		}
+	}
+
+}