theorem Th18:
  r <= s implies Ball(x,r) c= Ball(x,s)
proof
  reconsider xe = x as Point of Euclid n by TOPREAL3:8;
A1: Ball(x,r) = Ball(xe,r) by TOPREAL9:13;
  Ball(x,s) = Ball(xe,s) by TOPREAL9:13;
  hence thesis by A1,PCOMPS_1:1;
end;
