title: Incidence matrices
body: <p>Permutation of any two rows or columns in an incidence matrix simply corresponds to relabelling the vertices and edges of the same graph. Conversely, two graphs X and Y are isomorphic if and only if their incidence matrices A(X) and A(Y) differ only by permutations of rows and columns. </p>
  
  <p>Can someone explain me what does it mean, with an example. What exactly does "permutation of any two rows or columns" over hear means?</p>