reserve f for non constant standard special_circular_sequence,
  i,j,k,i1,i2,j1,j2 for Nat,
  r,s,r1,s1,r2,s2 for Real,
  p,q for Point of TOP-REAL 2,
  G for Go-board;

theorem Th17:
  1 <= k & k+1 <= len f implies Int left_cell(f,k) is convex
proof
  assume that
A1: 1 <= k and
A2: k+1 <= len f;
  ex i,j st i <= len GoB f & j <=
  width GoB f & cell(GoB f,i,j) = left_cell(f,k) by A1,A2,Th10;
  hence thesis by Th16;
end;
