The procedure until is not in the core, but you can define it yourself as follows:
proc until {cond code} { uplevel 1 [list while !($cond) $code] }
See also control::do (in tcllib).
PL: The original rewriting of the command was like this:
but that doesn't work since a cond value of, say, $i > 5 is rewritten as the condition argument !$i > 5 which will always evaluate to false (the ! operator has priority, and the result of evaluating it is either 1 or 0, which is never greater than 5). Other condition strings are likely to lead to other interesting errors.
Rewriting the command as
ensures that all of $cond is evaluated before negating the result.
This doesn't implement the usual meaning of until, which executes the code and then tests cond after.
DKF: The usual definition is typically expressed as a do-while loop, which is a form that puts the test last. When expressed condition-first, it is natural to expect the condition to be checked first...