function solve () {

	function isHTMLElement(obj){
		return obj === 'object' && obj instanceof HTMLElement;
	}

	function isString(obj){
		return obj === 'string' || obj instanceof String;
	}

	function isExistingId(id){
		var element = document.getElementById('id');
		return element !== 'undefine' && element !== null;
	}

	var validator = {
		validateParams: function (selector){
			if(!isString(selector) && !isHTMLElement(selector)){
				throw new Error('The provided id is not a string or does not select any DOM element!');
			}
		}

		validateExistingId: function (id){
			if (!isExistingId(id)) {
				throw new Error('The provided id does not select anything!');
			}
		}
	}

	return function (selector) {
		// Validations
		validator.validateParams(selector);
		validator.validateExistingId(selector);

		// Select all buttons and change their content to 'hide'
		var buttons = document.getElementsByClassName('button'),
			i,len;

		for(i = 0; len = buttons.length; i < len; i+=1){
			buttons[i].innerHTML = 'hide';
		}

		// Iterate over all buttons and implement onclick functionality
		for(i = 0; len = buttons.length; i < len; i+=1){
			var currButton = buttons[i];

			currButton.addEventListener('click', function(ev){
				var clickedButton = ev.target;
				var nextContent = clickedButton.nextElementSibling;
				while(nextContent && nextContent.className.indexOf('content') < 0){
					nextContent = nextContent.nextElementSibling;
				}

				var isContentVisible = nextContent.style.display === ''; // <=> display:block
				if (isContentVisible) {
					nextContent.style.display = 'none';
					clickedButton.innerHTML = 'show';
				}else{
					nextContent.style.display = ''; // <=> display:block
					clickedButton.innerHTML = 'hide';
				}

			});

		}
	}
}