reserve p, q for FinSequence,
  X, Y, x, y, e for set,
  D for non empty set,
  i, j, k, l, m, n, r for Nat;
reserve G for Graph;
reserve v, v1, v2, v3, v4 for Element of G;

theorem
  e joins v1,v2 & e joins v3,v4 implies v1=v3 & v2=v4 or v1=v4 & v2=v3;
