code: 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)
              }
docs: Computed all expression and get concated result