reserve i,j,k,k1,k2,n,m,i1,i2,j1,j2 for Element of NAT,
  x for set;

theorem
  for i1,i2,j1,j2 being Nat
   st i1,j1,i2,j2 are_adjacent & 1<=i1 & 1<=i2 & 1<=j1 &
  1<=j2 holds i1-'1,j1-'1,i2-'1,j2-'1 are_adjacent
by Th2;
