

          loop var first last [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 var argument is the name of a Tcl variable that
               will contain the loop index.  The loop index is first
               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 last, 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, last 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.
