The iterative decoding of a certain class of error correcting codes, namely LDPC codes, works best if the underlying graph structure locally looks like a tree. The bigger the tree-like vicinity of the vertex in the graph, the better the guaranteed performance of the iterative decoding. I was trying to come up with an error correcting code of 183 bits and a certain local tree structure. This was proved to be impossible because of a certain fact from finite projective geometry.