Idea
: some objects connected by some morphisms in a category
we draw diagrams by drawing some objects (or dots labeled by objects) connected by arrows labeled by morphisms.
the concept is often used when talking about (co)limits - we say “the limit or colimit of a diagram”

two natural ways to formally define diagram
- a functor, with a (very) small category as domain
- a functor, with a free category as domain
- we draw a directed graph, it freely generates a category
Definition
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