Index on a function(field)

Hi,

Is it possible to create an index using a function(field) sintaxis ?

For instance:

CREATE  INDEX "i1_cdu" on "cdu" using btree ( substr(cdu_code,1,1)
"varchar_ops" );

If not, should I alter the table to include a field with the value
'substr(codigo,1,1)'.

Thanks

Gabi :-)