[markegli]

The tiles in an aperiodic tiling must guarantee that the tiling is not periodic simply by their shape.

https://en.wikipedia.org/wiki/Aperiodic_tiling

All three of the Scheme examples can be tiled periodically even though they aren't tiled periodically in the examples. How to tile them periodically is left as an exercise to the reader, but it's not hard.

[GregBuchholz]

Got to love this quote from the "Einstein Problem" page on Wikipedia:

https://en.wikipedia.org/wiki/Einstein_problem

"Depending on the particular definitions of nonperiodicity and the specifications of what sets may qualify as tiles and what types of matching rules are permitted, the problem is either open or solved."

[Infernal]

There's definitely something Douglas Adams-esque about that phrasing.