make no value object serializable for use in model checker project

src/main/java/org/rebecalang/compiler/modelcompiler/abstractrebeca/AbstractSemanticCheck.java

@@ -6,6 +6,8 @@
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.stereotype.Component;
 
+import java.io.Serializable;
+
 @Component
 public abstract class AbstractSemanticCheck {
 	@Autowired
@@ -17,6 +19,14 @@ public abstract class AbstractSemanticCheck {
 	@Autowired
 	protected SymbolTable symbolTable;
 
-	public final static Object NO_VALUE = new Object();
+	public final static Object NO_VALUE = new NoValue();
+
+	private static class NoValue implements Serializable {
+		private static final long serialVersionUID = 1L;
 
+		// Ensure singleton instance
+		private Object readResolve() {
+			return NO_VALUE;
+		}
+	}
 }