code: function _update(kp, nv, pv, method, ind, len) {
          if (d.$destroyed) return
  
          var nexv
          if (isMultiExpr) {
              var lastV
              nexv = expr.map(function (exp, i) {
                  if (multiExprMetas[i][0]) {
                      if (lastV && isExclusion) return false
                      return (lastV = _exec(multiExprMetas[i][1]))
                  } else {
                      return exp
                  }
              })
          } else {
              nexv = _exec(expr)
          }
          var deltaResult 
          if ( delta && (deltaResult = delta.call(d, nexv, prev, kp)) ) {
              return deltaUpdate && deltaUpdate.call(d, nexv, prev, kp, deltaResult)
          }
          if (util.diff(nexv, prev)) {
              var p = prev
              prev = nexv
              upda && upda.call(d, nexv, p, kp, method, ind, len)
          }
      }
docs: update handler