create function syntax error

Hello

I Always get "syntax error" for every function that I try to create, for
example:
    mydb'>create or replace function f_sum () returns void as '
    mydb'> select 2+2;
    mydb'>  ' LANGUAGE plpgsql;
    ERROR:  syntax error at or near "select" at character 56
    LINE 2: select 2+2;
        ^

What its the problem ??
   Thank you,
         Gustavo