abstract class 'does not implement interface member ...'

I get
C:\Programming\LTM\devtools\UselessJunkForDissassembly\Class1.cs(360,27):
error CS0535: 'UselessJunkForDissassembly.InvocableInternals' does not
implement interface member
'UselessJunkForDissassembly.IInvocableInternals.OperationValidate(string)'
C:\Programming\LTM\devtools\UselessJunkForDissassembly\Class1.cs(360,27):
error CS0535: 'UselessJunkForDissassembly.InvocableInternals' does not
implement interface member
'UselessJunkForDissassembly.IInvocableInternals.ProxiedOperation'

when compiling:
public interface IInvocable
{
object Operation { get; }
}
internal interface IInvocableInternals : IInvocable
{
bool OperationValidate(string args);
string ProxiedOperation { get; }
}
public abstract class InvocableInternals : IInvocableInternals
{
public object Operation { get { return ProxiedOperation; } }
}

But, I already knew the class didn't implement those functions. That's why
it is *abstract*. Please note that I've replaced all complicated types with
object or string to make a minimal reproduction. I don't want my internal
functions exposed publicly, I can't hide InvocableInternals because public
classes inherit from it, and I don't want to use a forwarder because, I'm
convinced that the JIT wouldn't be able to inline it.
Why isn't it allowed to just implement
"IInvocableInternals.OperationValidate" in the most derived class?


.