More aggregate functions?

What do people think of adding some more aggregate functions. These are
the ones that MySQL has and PG doesn't:

- STD/STDDEV
- VARIANCE
- BIT_OR
- BIT_AND
- GROUP_CONCAT (for strings, added in MySQL 4.x)

Particularly, I find GROUP_CONCAT practical to quickly display 1-many
relationship, e.g.:

CREATE TABLE mommy (
id INT PRIMARY KEY,
name TEXT);
CREATE TABLE child (
id INT PRIMARY KEY,
mommy_id INT REFERENCES(mommy(id)),
name TEXT
);

SELECT
mommy.name as mommy,
GROUP_CONCAT(child.name SEPARATOR ", ") as children
FROM mommy, child
GROUP BY mommy.id;

would result in:

mommy children
----- -----------------
dot ellen, catherine
cate bridget, kerry, rory
rachel emma

Btw, I have written 1 or 2 of the above myself with CREATE AGGREGATE.
But perhaps it would be nice if it's available as a builtin function or
at least present in contrib/.

--
dave