'Tests if given orbit is only above fermi surface.
If fermi level has not been set we return True.
If nothing can be concluded we return a conservative False.'

@classmethod
def _only_above_fermi(cls, i):

    if i.is_number:
        return (i > cls.fermi_level)
    if i.assumptions0.get('above_fermi'):
        return True
    return (not cls.fermi_level)
