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).
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...