       lvarpop var ?indexExpr? ?string?
              The  lvarpop  command  pops  (deletes)  the element
              indexed by the expression indexExpr from  the  list
              contained   in  the  variable  var.   If  index  is
              omitted,  then  0  is  assumed.   If   string,   is
              specified,  then the deleted element is replaced by
              string.  The  replaced  or   deleted   element   is
              returned.   Thus  ``lvarpop  argv  0''  returns the
              first element of argv, setting argv to contain  the
              remainder of the string.

              If  the expression indexExpr starts with the string
              end, then end is replaced with  the  index  of  the
              last element in the list.  If the expression starts
              with len, then len is replaced with the  length  of
              the list.
