error when using multiple classes in one source file

Demo7.java

package com.github.cc007;

import lombok.ToString;
import manifold.ext.props.rt.api.set;
import manifold.ext.props.rt.api.var;

import static manifold.ext.props.rt.api.PropOption.Package;
import static manifold.ext.props.rt.api.PropOption.Private;
import static manifold.ext.props.rt.api.PropOption.Protected;

/**
 * With manifold-props, you can also specify if a setter should be private or protected.
 */
@ToString
class Properties {
    @var @set(Protected) public String protectedSet = "can only set in class and subclasses and other classes in the same package";
    @var @set(Package) String packagePrivateSet = "can only set in class and other classes in the same package";
    @var @set(Private) String privateSet = "can only set in class itself";

    public Properties() {
        protectedSet = "protected setter used in Properties";
        packagePrivateSet = "package-private setter used in Properties";
        privateSet = "private setter used in Properties";
    }
}

class SubProperties extends Properties {
    public SubProperties() {
        protectedSet = "protected setter used in SubProperties";
        packagePrivateSet = "package-private setter used in SubProperties";
    }
}

public class Demo7 {
    public static void main(String[] args) {
        Properties properties = new Properties();
        System.out.println(properties);
        SubProperties subProperties = new SubProperties();
        System.out.println(subProperties);
    }
}