st: how to create a new column

[west--@libero.it]

Consider these three columns, a, b, c:


a    b    c    | d
------------------

1    .    .    | 1

.    .    5    | 5

.   0.2   .    | 0.2 

2    .    .    | 2


where "." mean no data. Each line has only a box different 
from "."? Starting from the first three columns, I would 
like to form a new column "d", as in the example above. Please, help me with the commands. Many thanks!


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