2. Template user syntax

• 
  Stack< int > stack_i;
  Stack< double > stack_d;
  Stack< Stack < Rational > > stack_r; // be careful about spacing!
  
  stack_i.push( 10 );
  stack_d.push( 3.14 );
  stack_d.push( stack_i.pop() );
  Rational rat( 4, 5 );
  stack_i.push( rat ); // type safety??