flatten/2…where is it?

[Sean Charles]

I just renamed a predicate "my flatten" to "flatten" and got this!

    native code procedure flatten/2 cannot be redefined (ignored)

Yet on the documentation I cannot find flatten/2 anywhere at all so is that an omission from the documentation or have I done something wrong again?

On this page:

The word flatten appears once and once only in section 10.4.1.

Thanks,

Sean.