create or replace procedure sum_abc (a in int, b in int, c out int) as
begin
  c := a + b;
end;
/

show errors;
-- bind variable, to receive returned value
-- Note: can't put comment on the following line!
-- (apparently because it's a SqlPlus command, not SQL)
var x number;
call sum_abc(10,20,:x);
print :x

set serveroutput on;

declare
  sum2 number;  -- comment OK here
begin
  sum_abc(10, 20, sum2);  -- comment OK here
  dbms_output.put_line('sum is '||sum2);
end;
/
