M Theory Lesson 21
After a meandering introduction, mostly written for topologists, Leinster begins with a picture of an arrow for a multicategory:Of course, this is just the kind of picture that computer scientists like to draw, and indeed many computer scientists work with multicategories. One way of thinking of an operad is as a multicategory with one object.
In operad land, only boundaries really matter. A cylinder, for instance, changes into and a pair of pants into the face diagram