Version-aware psql?

Kind people,

I've been digging through the CVS code and trying to figure out the
best approach to making one psql that "does the right thing" for
different versions of the back-end. This has come up in a real
situation already, where the dev box is 7.3.3 and the production box
is (Yikes!) 7.2.1. Connecting from the 7.3.3 side results in a lot of
stuff like this:

Password:
ERROR: parser: parse error at or near "."
Welcome to psql 7.3.3, the PostgreSQL interactive terminal.

Type: \copyright for distribution terms
\h for help with SQL commands
\? for help on internal slash commands
\g or terminate with semicolon to execute query
\q to quit

template1=> \dt
ERROR: parser: parse error at or near "."
template1=>

Luckily, it works (so far) ok going from 7.2.1 -> 7.3.3, though.

So which files need consideration to make this reasonably seamless?
Are they all in src/bin/psql, or are there others elsewhere that
should know?

I'm wondering if maybe the Right Thing(TM) might be for all
connections to return (optionally?) the version number, but that seems
like a giant project with many backward-compatibility snags attached.

TIA for any tips, hints or pointers on this :)

Cheers,
D
--
David Fetter david(at)fetter(dot)org http://fetter.org/
phone: +1 510 893 6100 cell: +1 415 235 3778