No explanation of the modifier keyword

[billybonks]

https://tutorial.glimdown.com/3-event-handling/5-conditional-event-handling?showAnswer=1

prior to this example we have been using on by itself all of a sudden we need to use it with modifier also modifier seems to work without an import

[billybonks]

found an explanation https://tutorial.glimdown.com/6-partial-application/3-modifier