Declare Cursor...

I am trying to declare a cursor inside a plpgsql function in a
begin end block as in the following:
create function func_1(char, float8, int4, int4) returns int4 as '
declare
x alias for $1;
y alias for $2;
z alias for $3;
t alias for $4;
...
begin work;
declare cursor_x cursor for select a, b, c, d from table_1 where a = x;
...
end work;

when this function is compiled, it gives me the following error:
NOTICE: plpgsql: ERROR during compile of func_1 near line ...
ERROR: parse error at or near "CURSOR"

If anyone can help me about my problem, I will be very happy...THANKS!

Baris ULU
Computer Engineer