          for start test next body
               For is a looping command, similar in structure to the C
               for  statement.   The  start,  next, and body arguments
               must be Tcl command strings, and test is an  expression
               string.    The   for  command  first  invokes  the  Tcl
               interpreter  to  execute  start.   Then  it  repeatedly
               evaluates  test as an expression; if the result is non-
               zero it invokes  the  Tcl  interpreter  on  body,  then
               invokes  the  Tcl interpreter on next, then repeats the
               loop.  The command terminates when test evaluates to 0.
               If  a  continue command is invoked within body then any
               remaining commands in the current execution of body are
               skipped;  processing  continues  by  invoking  the  Tcl
               interpreter on next, then evaluating test, and  so  on.
               If a break command is invoked within body or next, then
               the for command will return immediately.  The operation
               of  break and continue are similar to the corresponding
               statements in C.  For returns an empty string.
