SQL DDL: FOREIGN KEY construct and field mapping: unexpected behavior

Hello,

Consider the following code run under PostgreSQL 7.3.4:

CREATE TABLE X ( A INT, B INT, PRIMARY KEY (A, B)
);

CREATE TABLE Y ( A INT, B INT, C INT, PRIMARY KEY (C), FOREIGN KEY (B, A) REFERENCES X
);

INSERT INTO X (A, B) VALUES (1, 2);
INSERT INTO Y (A, B, C) VALUES (1, 2, 3);

The second insert causes the database server to report the following
error:
ERROR:  $1 referential integrity violation       - key referenced from y not found in x

Upon examining the interpretation of PostgreSQL's
REFERENCES clause I find the following:

foodb=# \d x      Table "public.x"Column |  Type   | Modifiers
--------+---------+-----------a      | integer | not nullb      | integer | not null
Indexes: x_pkey primary key btree (a, b)

foodb=# \d y      Table "public.y"Column |  Type   | Modifiers
--------+---------+-----------a      | integer |b      | integer |c      | integer | not null
Indexes: y_pkey primary key btree (c)
Foreign Key constraints: $1 FOREIGN KEY (b, a) REFERENCES x(a, b) ON UPDATE NO ACTION ON DELETE NO ACTION

It is as though PostgreSQL, instead of matching names, associated

field A in table Y with field B in table X
and
field B in table Y with field A in table X

whereas I was expecting the database server to match the names as in:

field A in table Y with field A in table X
and
field B in table Y with field B in table X

I wonder what the SQL standard has to say on this one and how the
REFERENCES clause with no field names on the right hand side
really works in spite of the unexpected results produced by
this very simple example...

Thanks,

Neil