one of my early 'let's learn scala' projects: some basic representation and validaion of pasting schemes in 2-categories
represents 2-categories using a graphviz 'dot' style language, with some conventions on how to treat objects, 1-cells and 2-cells.
Also has some restrictions to (attempt to) ensure that only valid pasting schemes are representable.
Apart from providing an embarrassing example of early scala-neophyte code from someone used to java, it is a reasonable example of the ease of testing parser combinators.