old_contents: describe('the stack spec', () => {
    it('shows the infrastructure works', () => {
      true.should.be.true();
    });
  
    const stack = {
      isEmpty: () => true,
      size: () => {}
    };
  
    describe('a stack should', () => {
      it('start empty', () => {
        stack.isEmpty().should.be.true();
      });
  
      it('start with stack size 0', () => {
        stack.size().should.equal(0);
      });
  
      it('not be empty when pushed');
  
      it('leave stack size 1 when pushed');
  
      it('leave stack empty when pushed and popped');
  
      it('leave stack size 0 when pushed and popped');
  
      it('overflow');
  
      it('underflow');
  
      it('pop the same one pushed');
  
      it('pop the same two pushed');
  
      it('accept only positive capacity');
    });
  });
  
new_contents: describe('the stack spec', () => {
    it('shows the infrastructure works', () => {
      true.should.be.true();
    });
  
    const stack = {
      isEmpty: () => true,
      size: () => null
    };
  
    describe('a stack should', () => {
      it('start empty', () => {
        stack.isEmpty().should.be.true();
      });
  
      it('start with stack size 0', () => {
        stack.size().should.equal(0);
      });
  
      it('not be empty when pushed');
  
      it('leave stack size 1 when pushed');
  
      it('leave stack empty when pushed and popped');
  
      it('leave stack size 0 when pushed and popped');
  
      it('overflow');
  
      it('underflow');
  
      it('pop the same one pushed');
  
      it('pop the same two pushed');
  
      it('accept only positive capacity');
    });
  });
  
subject: change message to TypeError: Cannot read property 'should' of null