Dropping a Primary Key using a named Unique Index in Oracle

-- TESTING THE BEHAVIOUR OF ORACLE WHEN DROPPING A PRIMARY KEY CONSTRAINT USING A NAMED UNIQUE INDEX
-- ORACLE DOCUMENTATION SAYS THE FOLLOWING:
-- When you drop a primary key constraint:
--  If the primary key was created using an existing index, then the index is not dropped.
--  If the primary key was created using a system-generated index, then the index is dropped.

-- CREATING A TABLE WITH A PK CONSTRAINT THAT USES A NAMED UNIQUE INDEX
CREATE TABLE dummy_table (a int, b CHAR);
CREATE UNIQUE INDEX dummy_ndx ON dummy_table(a);
ALTER TABLE dummy_table ADD CONSTRAINT dummy_pk PRIMARY KEY (a) USING INDEX dummy_ndx;

-- POPULATING THE TABLE WITH VALUES
INSERT INTO dummy_table VALUES (1, 'a');
INSERT INTO dummy_table VALUES (2, 'b');
INSERT INTO dummy_table VALUES (3, 'c');

-- ALTERING THE TABLE BY DROPPING THE PRIMARY KEY CONSTRAINT
ALTER TABLE dummy_table DROP CONSTRAINT dummy_pk;

-- ATTEMPTING TO INSERT RECORDS AFTER THE PK HAS BEEN DROPPED
INSERT INTO dummy_table VALUES (4, 'd'); -- this should work
INSERT INTO dummy_table VALUES (2, 'm'); -- this triggers a unique constraint error