How to move one file record format to another

["Jack Tucky" <jacktucky@xxxxxxxxx>]

I recall there was a way to create a data structure to make one files fields
to another using a data structure?  I can't figure out how it's done.

Basically I have two files that have the same field, except a few are
different lengths.  All signed/packed/character definitions match.

So we did this:

FFile1     uP   E           K DISK                
f                                      prefix(x_)  
FFile2     if   E           K DISK                

But then we have to eval each field to the other.  

I remember that there is a way to setup a data structure so that all the
fields that are prefixed as X can automatically be put into the fields
without the prefix.  Can someone show me how to do that?  Thanks

Jack