reserve X for set;
reserve CS for non empty CollStr;
reserve a,b,c for Point of CS;
reserve CLSP for CollSp;
reserve a,b,c,d,p,q,r for Point of CLSP;
reserve i,j,k for Element of NAT;
reserve CLSP for proper CollSp;
reserve a,b,c,p,q,r for Point of CLSP;
reserve P,Q for LINE of CLSP;

theorem Th20:
  p<>q & p in P & q in P & p in Q & q in Q implies P = Q
proof
  assume that
A1: p<>q and
A2: p in P & q in P and
A3: p in Q & q in Q;
  Line(p,q) = P by A1,A2,Th19;
  hence thesis by A1,A3,Th19;
end;
