SQL & PL/SQL :: ORA-04091 After Insert Trigger?

In

CREATE table ALPHA (
ID NUMBER,
FK_NR NUMBER,
INPUT NUMBER,
OUTPUT NUMBER,
MY_COUNT NUMBER,
ACTUAL_TOTAL NUMBER
);

I would like to record new inputs or outputs andthe column MY_COUNT should display the actual count of records for INPUT (actually MOD(INPUT, 10) ) regarding FK_NR and ACTUAL_TOTAL should display the actual sum for all input/output for FK_NR.

--ID normally comes from a sequence and BEFORE INSERT TRIGGER

EXAMPLE
INSERT INTO ALPHA(FK_NR, INPUT, OUTPUT)
VALUES (1 ,10,NULL);
INSERT INTO ALPHA(FK_NR, INPUT, OUTPUT)
VALUES (1 ,10, 50);

[code]...

Table ALPHA looks like:
1,1,10,NULL,1,10
2,1,10,50,2,-30
3,2,NULL,20,0,-20
4,2,20,40,2,-40
5,3,40,20,4,20
6,3,-10,NULL,3,10

How can this be achieved ?I started with a

CREATE OR REPLACE
TRIGGER "BI_ALPHA"
BEFORE INSERT ON ALPHA
FOR EACH ROW
BEGIN
SELECT ALPHA_SEQ.nextval INTO :NEW.ID FROM DUAL;
END;

and thought of a

CREATE OR REPLACE TRIGGER "AI_ALPHA"
AFTER INSERT ON "ALPHA"
FOR EACH ROW
DECLARE
l_maxidNUMBER;
l_fknr NUMBER;

[code]...

But by inserting a row, this results in ORA-04091