       csubstr string firstExpr lengthExpr
              Returns  a range of characters from string starting
              at the character indexed by the  expression  first-
              Expr (zero-based) for lengthExpr characters.

              If  the  expression  firstExpr or lengthExpr 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.
