generic crosstab

I'm using the PostgreSQL crosstab functions to do something like this:

From this table ...

a    x     10
b    x     13
a    y     11
b    y     17


... I get:

          x        y
a        10       11
b        13       17


To get the second table I have to specify the field list when I call the crosstab function (in this example it would be
ASt(x integer, y integer)) 
My problem arises when that field list changes, for example when someone inserts a record in the first table with a new
value,for example "z". In that case I have to change my queries to AS t(x integer, y integer, z integer) 

Do you know how to declare a "generic" field list, or some other library similar to crosstab?

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