Returns or updates element value. If such element doesn't exists,
it will be created automatically and added at the end of child list.
@param {String} name Element name or its index
@param {String} value New element value
@returns {String}
function(name, value, pos) {
			var element = this.get(name);
			if (element)
				return element.value(value);
			
			if (!_.isUndefined(value)) {
				// no such element — create it
				return this.add(name, value, pos);
			}
		}