       crange string firstExpr lastExpr
              Returns  a range of characters from string starting
              at the character indexed by the  expression  first-
              Expr  (zero-based)  until  the character indexed by
              the expression lastExpr.

              If the expression firstExpr or lastExpr starts with
              the string end, then end is replaced with the index
              of the  last  character  in  the  string.   If  the
              expression  starts  with  len, then len is replaced
              with the length of the string.

              This command is provided by Extended Tcl.
