# The Challenge

Consider the 3x3 king grid, as shown in the following ASCII graphic:

    A--B--C
    |\/|\/|
    |/\|/\|
    D--E--F
    |\/|\/|
    |/\|/\|
    G--H--I

You are given as input a length-9 list of integers that represent a labeling of the nodes. For example, the input `[0,1,1,2,1,0,5,5,1]` represents the following labeling:

    0--1--1
    |\/|\/|
    |/\|/\|
    2--1--0
    |\/|\/|
    |/\|/\|
    5--5--1

Your output is the set of integers in the input that form connected sets of nodes. More explicitly, the output should contain an integer `n` from the input if and only if the set of nodes with label `n` is connected. In this example, an acceptable output would be `[1,2,5]`, since the two `0`s are not connected. The lowest byte count wins.

# Detailed rules

- You can choose a **fixed ordering** for the nodes in your input list, and you should state this in your answer. In the order EFBDHCAGI, the above labeling would be given as `[1,0,1,2,5,1,0,5,1]`.
- You can write either a full program or a function. In the latter case, the output can be a set of integers if your language supports those.
- The output list may contain duplicates, but its length must not exceed 9.
- Standard loopholes are disallowed.