       loop var first limit ?increment? body
              Loop  is  a looping command, similar in behavior to
              the Tcl for statement, except that the loop  state-
              ment  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.

              The  first, limit and increment are integer expres-
              sions.  They are only evaluated once at the  begin-
              ning of the loop.

              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.

              This command is provided by Extended Tcl.
