# load source code into $_
    while( m{['"{}]|//|/*|\bclass\s+}g ) {
        my $c = substr( $_, pos($_)-1, 1 );
        if( "'" eq $c ) {
            m{\G(?:[^\\']+|\\.)*'}gc
                or  fail( \$_, "Unclosed char" );
        } elsif( '"' eq $c ) {
            m{\G(?:[^\\"]+|\\.)*"}gc
                or  fail( \$_, "Unclosed string" );
        } elsif( '/' eq $c ) {
            m{\G[^\n]*}gc;
        } elsif( '*' eq $c ) {
            m{*/}gc
                or  fail( \$_, "Unclosed comment" );
        } elsif( '{' eq $c ) {
            $nest++;
        } elsif( '}' eq $c ) {
            $nest--;
        } elsif( 's' eq $c ) {
            my $p = pos();
            m{\G\w+}gc
                or  fail( \$_, "'class' not followed by ID" );
            my $name = substr( $_, $p, pos()-$p );
            if( m[\G\s*:\s*(?:\w+\s+)?(\w+)\s+{]gc ) {
                my $ancestor = $1;
                $nest++;
            } elsif( ! m[\s*{}gc ) {
                fail( \$_, "No { after 'class $name'" );
            }
        }
    }