title: Open Refine--create new column by looking up values from a pair of columns
body: <p>I have a table in OpenRefine with columns A, B, and C like this:</p>
  
  <p><code>
   A | B | C | D 
  ---|---|---|---
   a | 1 | b | 2
   b | 2 |   |
   c | 3 | a | 1
   d | 4 | c | 3
  </code>
  I want to create a column D by fetching the values from B corresponding to those in C, using A as an index. Hope that makes sense? I'm not having much luck figuring out how to do this in GREL.</p>