# usage @perm = &permutation ( $Q , 0 ) ;
sub permutation { # returns a permn of 0..Q offset by offset eg 0/1
    local ( $Q , $offset ) = @_ ;
    for ( local($q) = 0 ; $q <= $Q ; $q ++ ) { $qs[$q] = $q + $offset ; }
    for ( local($n) = 0 ; $n <= $Q ; $n ++ ) {
#       $from = $n ;
        $q = int ( rand($#qs+1) ) ;
        $to[$n] = splice ( @qs , $q , 1  ) ;
        print "to[$n] = $to[$n]\n" if ( $verbose >=3 ) ; 
    }
    print "Here is a perm $Q:\n"  if ( $verbose >=3 ) ; 
    print join ( ':' ,  @to  ) , "\n" if ( $verbose >=3 )  ; 
    return ( @to ) ; 
}