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

theorem
  e orientedly_joins v1,v2 & e orientedly_joins v3,v4 implies v1=v3 & v2=v4;
