Computed all expression and get concated result
function compute () {
                // set value
                util.forEach(expressions, function(exp, index) {
                    var v = that.$exec(exp)
                    if (!v[0]) caches[index] = v[1]
                })
                // get content
                var str = ''
                util.forEach(parts, function(item, index) {
                    str += emptyStr(item)
                    if (index < expressions.length) {
                        str += emptyStr(caches[index])
                    }
                })
                return Expression.unveil(str)
            }