How to find item between two items in list?

Question

I have the following list

a = [[\'-\',\'k\',\'-\',\'x\',\'k\',\'x\',\'-\', \'k\'],
     [\'k\',\'k\',\'-\',\'x\',\'k\',\'-\',\'-\', \'k\'],
     [\'k\',\'-\',\'-\