reserve X,Y,Z,x,y,z for set;
reserve T,R for Tolerance of X;

theorem Th18:
  Y is TolSet of T implies Y c= X
proof
  assume
A1: Y is TolSet of T;
  let x be object;
  assume x in Y;
  then [x,x] in T by A1,Def1;
  hence thesis by Th7;
end;
