I am trying to combine two columns in a data frame into one column.  Some
values in either column are missing, but not in the same row for the two
different columns.  Additionally, when both columns in a row contain data,
the data are identical.  I want a new column with the identical data or the
data from the column with observed data.  For example:

I have
>data
           id   x    y
       1   a   1   NA
       2   b   2    2
       3   c   3    3
       4   d  NA  4

And I want
>new.data
           id   x    y     z
       1   a   1   NA   1
       2   b   2    2     2
       3   c   3    3     3
       4   d  NA  4     4

I've looked through the help and there are column combining solutions, but
they don't seem to work well for this solution.
Thanks for any help!
Andy

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