reserve AS for AffinSpace;
reserve a,a9,b,b9,c,d,o,p,q,r,s,x,y,z,t,u,w for Element of AS;
reserve A,C,D,K for Subset of AS;

theorem Th18:
  A is being_line implies ex a,b st a in A & b in A & a<>b
proof
  assume A is being_line;
  then consider a,b such that
A1: a<>b and
A2: A=Line(a,b);
A3: b in A by A2,Th14;
  a in A by A2,Th14;
  hence thesis by A1,A3;
end;
