+ function has been done

src/main/java/serene/simple/SNumber.java

@@ -0,0 +1,79 @@
+package serene.simple;
+
+import serene.simple.SereneException;
+
+
+public class SNumber {
+  public static interface IOps {
+    public IOps add(Object x);
+    public IOps add(Double x);
+    public IOps add(Long x);
+
+    public Object value();
+  }
+
+  public static class LongNumber implements IOps  {
+    private long v;
+
+    public LongNumber() {
+      this.v = 0;
+    }
+
+    public LongNumber(long v) {
+      this.v = v;
+    }
+
+    public IOps add(Object x) throws SereneException {
+      if (x instanceof Long) {
+        return this.add((long) x);
+      }
+      else if (x instanceof Double) {
+        return this.add((Double) x);
+      }
+      throw new SereneException("Can't cast anything beside Long and Double");
+    }
+
+    public IOps add(Long x) {
+      return new LongNumber(this.v + x);
+    }
+
+    public IOps add(Double x) {
+      DoubleNumber y = new DoubleNumber(x);
+      return y.add(this.v);
+    }
+
+    public Object value() {
+      return this.v;
+    }
+  }
+
+  public static class DoubleNumber  implements IOps  {
+    private Double v;
+
+    public DoubleNumber(Double v) {
+      this.v = v;
+    }
+
+    public IOps add(Object x) throws SereneException {
+      if (x instanceof Long) {
+        return this.add((long) x);
+      }
+      else if (x instanceof Double) {
+        return this.add((Double) x);
+      }
+      throw new SereneException("Can't cast anything beside Long and Double");
+    }
+
+    public IOps add(Double x) {
+      return new DoubleNumber(this.v + x);
+    }
+
+    public IOps add(Long x) {
+      return new DoubleNumber(this.v + x.doubleValue());
+    }
+
+    public Object value() {
+      return this.v;
+    }
+  }
+}

src/main/java/serene/simple/SereneException.java

@@ -1,7 +1,7 @@
 package serene.simple;
 
 
-class SereneException extends RuntimeException {
+public class SereneException extends RuntimeException {
     private static final long serialVersionUID = 1L;
 
     public SereneException(String message) {

src/main/java/serene/simple/builtin/PlusFn.java

@@ -1,6 +1,8 @@
 package serene.simple.builtin;
 
 import serene.simple.BaseScope;
+import serene.simple.SNumber;
+import serene.simple.SereneException;
 
 
 public class PlusFn extends AFn {
@@ -8,7 +10,13 @@ public class PlusFn extends AFn {
     return "+";
   };
 
-  public Object eval(BaseScope scope) {
-    return 0;
+  public Object eval(BaseScope scope) throws SereneException{
+    SNumber.IOps result = (SNumber.IOps) new SNumber.LongNumber();
+
+    for(Object x: this.arguments()) {
+      result = (SNumber.IOps) result.add(x);
+    }
+
+    return result.value();
   }
 }