reserve i,j,k,k1,k2,i1,i2,j1,j2 for Nat,
  r,s for Real,
  x for set,
  f for non constant standard special_circular_sequence;

theorem Th1:
  for i,j st i<=len GoB f & j<=width GoB f holds Int cell(GoB f,i,j )c=(L~f)`
    by GOBOARD7:12,SUBSET_1:23;
