  We propose that an extension of the exciton concept to doped Mott insulators
offers a fruitful insight into challenging issues of the copper oxide
superconductors. In our extension, new fermionic excitations called cofermions
emerge in conjunction to generalized excitons. The cofermions hybridize with
conventional quasiparticles. Then a hybridization gap opens, and is identified
as the pseudogap observed in the underdoped cuprates. The resultant
Fermi-surface reconstruction naturally explains a number of unusual properties
of the underdoped cuprates, such as the Fermi arc and/or pocket formation.
