It is becoming essential classroom practice for teachers to warn students that something in that day's lesson might trigger anxiety, or even PTSD. Trigger warnings are "pre-emptive alerts" given by professors about material that could "trigger" extreme discomfort - even PTSD.

The idea first appeared in the late 90s. But by 2014, pushed by student groups, some U.S. universities began drafting guidelines. Professors were advised to "avoid triggering material", And to be "aware of racism, classism, sexism, heterosexism, cissexism, ableism, and other issues of privilege and oppression."

Suddenly students wanted Ovid's Metamorphosis to have trigger warnings because of its rape scenes. Chinua Achebe's novels might upset readers who have experienced racism, colonialism, or suicide. Before you could say, "There goes the literary canon," trigger warnings were fuelling a fierce debate. It is a hot topic in Canadian universities - nowhere more than in gender and women's studies departments.

Producer Frank Faulk's documentary is called "Trigger Happy".