def get_stackless_package():
    """
    Compute the required Stackless-Python package.
    
    This method returns a string pair (name, version-specifier). 
    If the package :mod:`stackless` is already available, the method
    returns (None,None). 
    """
    from distutils.util import get_platform
    from distutils import log
    import sys
    try:
        import stackless
    except ImportError:
        pass
    else:
        log.info("Your Python version already provides the package 'stackless'"
            )
        return None, None
    reqVersionMin = '.'.join(map(str, sys.version_info[:3]))
    vi = list(sys.version_info[:3])
    vi[-1] += 1
    reqVersionMax = '.'.join(map(str, vi))
    reqVersion = '>=%s,<%s' % (reqVersionMin, reqVersionMax)
    ucs = 4 if sys.maxunicode >> 16 else 2
    platform = get_platform()
    if getattr(sys, 'pypy_version_info', None):
        raise RuntimeError('PyPy is not supported')
    elif platform.startswith('java'):
        raise NotImplementedError('Platform %s not yet supported' % (platform,)
            )
    installer = 'stackless_installer_C%d_%s' % (ucs, platform.replace('-', '_')
        )
    return installer, reqVersion
