There is a unwritten rule in presidential debates—and general conversation—that people should not interrupt each other. All too often in professional situations, men interrupt women.

The first US presidential debate in history to feature a woman a candidate has proven to be a textbook example of this practice. In 90 minutes, Donald Trump interrupted Hillary Clinton 51 times, often talking over her for several seconds.

Trump’s conduct during the debate exemplifies the type of microaggression that belittles women publicly as a way to assert power, and to dismiss women.

He interrupted, but hated being interrupted himself for going off topic during his replies, or taking up too much time, and he interrupted the host Lester Holt, too, several times—essentially using aggression to replace assertiveness.

Trump did more to make this debate a case-study in sexism by saying Clinton doesn’t have the “stamina” to be a president. That kind of coded language is a classic way of saying that she’s a woman, and therefore, weaker.

He criticized Clinton’s temperament, giving voice to the unfair perception that strong women are aggressive or angry. As he left, he patted Clinton on the back—another form of physical microaggression.

But the reason why this debate should go down in gender study history is also another: the way Clinton handled that.

Clinton was assertive, strong, and did not let her adversary’s aggressive behavior get to her. When he spoke over her, she kept delivering her message with confidence. Often, she turned his sexism back at him, using his offensive words to remind the audience of all the other times he directed ugly words toward women.

“I prepared to be president,” she said at the peak of the debate. And as the first woman to be running for the most powerful office in the world, she was a great role model for all women who find themselves in similar positions: she kept going, relying on her preparation and qualification, making Trump look every bit the bully that he is.

An earlier version of this article included a reference to a tweet about Trump taking up 62% of the “talk time” during the debate that was incorrect.

Correction: Donald Trump interrupted Hillary Clinton 51 times.