Problems in create table ?? 6.3.2

Hello:
I am getting the following errors. I am using 6.3.2 on linux Redhat 5.1

create table person (person_id int not null,
marital_status char(1),
unique(person_id));
NOTICE:  CREATE TABLE/UNIQUE will create implicit index
person_person_id_key for table person
CREATE
My comments: This notice looks ok.

create table longtablename (DATE_TIME_STAMP datetime not null,
UNIQUE (DATE_TIME_STAMP));
ERROR:  parser: unable to construct implicit index for table
longtablename; name too long
My comments: Here if the table name is long and the column name is long
Postgres has trouble with
the index name being too long.

ALTER TABLE longtablename ADD FOREIGN KEY (person_ID) REFERENCES
person(person_id);
NOTICE:  CREATE TABLE/FOREIGN KEY clause ignored; not yet implemented
ERROR:  ALTER TABLE/ADD CONSTRAINT not yet implemented
My comments: Here, since the table is empty, should this not be easy for
the postgres to alter the table??

ALTER TABLE person ADD
check (marital_status in ("T","F"))
;
ERROR:  parser: parse error at or near "in"
EOF
My comments: Here, since the table is empty, should this not be easy for
the postgres to alter the table??