What geometry theorem is dynamically being illustrated in the applet below?[br](Feel free to move the larger white points anywhere you'd like.)
[b]Theorem:[/b][br][br]If ONE PAIR OF [b][color=#9900ff]OPPOSITE SIDES[/color][/b] of a quadrilateral is [color=#9900ff][b]BOTH CONGRUENT[/b][/color][color=#38761d] [b]AND PARALLEL[/b][/color], then that quadrilateral is a parallelogram.