

          loop var first limit [increment] body
               Loop is a looping command, similar in behavior to the
               Tcl for statement, except that the loop statement
               achieves substantially higher performance and is easier
               to code when the beginning and ending values of a loop
               are known, and the loop variable is to be incremented
               by a known, fixed amount every time through the loop.

                The var argument is the name of a Tcl variable that
               will contain the loop index.  The loop index is set to
               the value specified by first.  The Tcl interpreter is
               invoked upon body zero or more times, where var is
               incremented by increment every time through the loop,
               or by one if increment is not specified.  Increment can
               be negative in which case the loop will count
               downwards.

               When var reaches limit, the loop terminates without a
               subsequent execution of body.  For instance, if the
               original loop parameters would cause loop to terminate,
               say first was one, limit was zero and increment was not
               specified or was non-negative, body is not executed at
               all and loop returns.

               If a continue command is invoked within body then any
               remaining commands in the current execution of body are
               skipped, as in the for command.  If a break command is
               invoked within body then the loop command will return
               immediately.  Loop returns an empty string.
