Indexing arrays

I'd like to index an array. This is my table:
template1=> \d test_array
Table    = test_array
+----------------------------------+----------------------------------+-------+
|              Field               |              Type                | Length|
+----------------------------------+----------------------------------+-------+
| name                             | text                             |   var |
| pay_by_quarter                   | int4[]                           |   var |
| schedule                         | text[]                           |   var |
+----------------------------------+----------------------------------+-------+

How can I create an index for schedule? I need to index every occurrence of this array.
Thanks. Stefano
--
Dott. Stefano Bargioni
Biblioteca della Pontificia Universita' della Santa Croce - Roma
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