  In a recent X-ray absorption study in boron doped diamond, Nakamura et al.
have seen a well isolated narrow boron impurity band in non-superconducting
samples and an additional narrow band at the chemical potential in a
superconducting sample. We interpret the beautiful spectra as evidence for
upper Hubbard band of a Mott insulating impurity band and an additional
metallic `mid-gap band' of a conducting `self-doped' Mott insulator. This
supports the basic framework of a recent theory of the present author of
strongly correlated impurity band superconductivity (SCIBS) in a template of a
wide-gap insulator, with no direct involvement of valence band states.
