core/src/com/google/inject/internal/RealMultibinder.java
public static <T> RealMultibinder<T> newRealSetBinder(Binder binder, Key<T> key) {
    binder = binder.skipSources(RealMultibinder.class);
    RealMultibinder<T> result = new RealMultibinder<>(binder, key);
    binder.install(result);
    return result;
  }
public static <T> RealMultibinder<T> newRealSetBinder(Binder binder, Key<T> key) {
    binder = binder.skipSources(RealMultibinder.class);
    RealMultibinder<T> result = new RealMultibinder<>(binder, key);
    binder.install(result);
    return result;
  }
Implementation of newSetBinder.