def GetBraviasNum(center,system):
    """Determine the Bravais lattice number, as used in GenHBravais
    
    :param center: one of: 'P', 'C', 'I', 'F', 'R' (see SGLatt from GSASIIspc.SpcGroup)
    :param system: one of 'cubic', 'hexagonal', 'tetragonal', 'orthorhombic', 'trigonal' (for R)
      'monoclinic', 'triclinic' (see SGSys from GSASIIspc.SpcGroup)
    :return: a number between 0 and 13 
      or throws a ValueError exception if the combination of center, system is not found (i.e. non-standard)

    """
    if center.upper() == 'F' and system.lower() == 'cubic':
        return 0
    elif center.upper() == 'I' and system.lower() == 'cubic':
        return 1
    elif center.upper() == 'P' and system.lower() == 'cubic':
        return 2
    elif center.upper() == 'R' and system.lower() == 'trigonal':
        return 3
    elif center.upper() == 'P' and system.lower() == 'hexagonal':
        return 4
    elif center.upper() == 'I' and system.lower() == 'tetragonal':
        return 5
    elif center.upper() == 'P' and system.lower() == 'tetragonal':
        return 6
    elif center.upper() == 'F' and system.lower() == 'orthorhombic':
        return 7
    elif center.upper() == 'I' and system.lower() == 'orthorhombic':
        return 8
    elif center.upper() == 'A' and system.lower() == 'orthorhombic':
        return 9
    elif center.upper() == 'B' and system.lower() == 'orthorhombic':
        return 10
    elif center.upper() == 'C' and system.lower() == 'orthorhombic':
        return 11
    elif center.upper() == 'P' and system.lower() == 'orthorhombic':
        return 12
    elif center.upper() == 'C' and system.lower() == 'monoclinic':
        return 13
    elif center.upper() == 'P' and system.lower() == 'monoclinic':
        return 14
    elif center.upper() == 'P' and system.lower() == 'triclinic':
        return 15
    raise ValueError('non-standard Bravais lattice center=%s, cell=%s' % (center,system))