sub PutEnvelope {
        my $self = shift; # obj or hashref
        my $tref = shift; # tag hashref to be consigned to envelope

        my $eref = ( $self -> { ESTACK } ||= [] ); # envelope stack
        my $tag = each %$tref;
        push @$eref,
             { $tag => { ASSMNTS  => $tref -> { ASSMNTS },
                         COMMENT => $tref -> { COMMENT },
                         DEPTH => $self -> { THIS }{ DEPTH } }
             };

        # note SUBTAGS are not stacked up
        if ( $self -> { THIS }{ DEPTH } == 1 ) {
                # stack now at top. Unwind it:-
                MakeEnvelope( $self, $self -> { ENVELOPE } ||= {} );
        }
}
 
sub MakeEnvelope { # recursively build envelope hash
        my ( $self, $eref ) = @_; # eref = where to build it

        $self -> { THIS }{ ENVDEPTH } ||= 0;
        $self -> { THIS }{ ENVDEPTH }++;
        my $frame = pop @{ $self -> { ESTACK }};
        my $tag = each %$frame;
 
        unless( $frame ) {
                $self -> { THIS }{ EBVDEPTH }--;
                return;
        }
        while( $frame -> { $tag }{ DEPTH }
             < $self -> { THIS }{ ENVDEPTH } ) {
                XMLerror( $self,
                          "multiple tags found at same "
                         ."level of envelope (i.e. tags "
                         ."above DEPTH) - ignoring tag $tag",
                          'WARNING'
                        );
                $frame = pop @{ $self -> { ESTACK } };
                unless( $frame ) {
                        $self -> { THIS }{ EBVDEPTH }--;
                        return;
                }
        }

        delete ( $frame -> { $tag => { DEPTH } } ); # done its job
        $eref -> { $tag } = $frame -> { $tag };

        if ( @{ $self -> { ESTACK } } ) {
                MakeEnvelope( $self,
                              $eref -> { $tag => { SUBTAGS }} [0] = {} );
                # make sub-envelope and put back SUBTAGS
        }
        $self -> { THIS }{ ENVDEPTH }--;
        return;
}