Replace substring of tag's source
@param {String} value
@param {Number} start
@param {Number} end
@private
function(value, start, end) {
			// create modification range
			var r = range(start, _.isUndefined(end) ? 0 : end - start);
			var delta = value.length - r.length();
			
			var update = function(obj) {
				_.each(obj, function(v, k) {
					if (v >= r.end)
						obj[k] += delta;
				});
			};
			
			// update affected positions of current container
			update(this._positions);
			
			// update affected positions of children
			_.each(this.list(), function(item) {
				update(item._positions);
			});
			
			this.source = require('utils').replaceSubstring(this.source, value, r);
		}