We were on vacation when I saw a pair of Z-coils for the first time. Had no idea what they were. They sure are interesting looking. I asked a granddaughter about the Sketchers; she hasn't
gotten back to me, yet. I'll ask one of her sisters. If I get an answer, I'll let ya know.
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https://www.amazon.com/author/bonnie.mill-lemke
https://kdp.amazon.com/en_US/bookshelf