overriding private interface methods

[Oleg]

Hello. How to override private methods?

import std.stdio, std.conv;
interface abcInterface
{
    private double private_func();
    public final double func() { return private_func(); }
}

class abcImpl: abcInterface
{
    override private double private_func() { return 3.14; } //#1
}

void main()
{
    auto abc = new abcImpl;
    writeln( abc.func() );
}

This code generate error
#1 Error: function inter.abcImpl.private_func cannot override a 
non-virtual function

This private method (private_func) is 'private' because it must 
call only in final interface methods (func)