code: @GwtIncompatible("To be supported")
    @Override
    MapMaker keyEquivalence(Equivalence<Object> equivalence) {
      checkState(keyEquivalence == null, "key equivalence was already set to %s", keyEquivalence);
      keyEquivalence = checkNotNull(equivalence);
      this.useCustomMap = true;
      return this;
    }
docs: Sets a custom {@code Equivalence} strategy for comparing keys.
  
  <p>By default, the map uses {@link Equivalence#identity} to determine key equality when {@link
  #weakKeys} is specified, and {@link Equivalence#equals()} otherwise. The only place this is
  used is in {@link Interners.WeakInterner}.