X and Y are for XSLT, yacc and Yorick.
XSLT is the tree climbing Kangaroo of the programming language world. Eating your own dog food is good practice for implementors, but users should not be forced to endure it. Anyway, people only use XML, rather than JSON, to increase the size of their files so they can claim to be working with big data.
yacc grammars don’t qualify as programming languages because they are not Turing complete (because yacc requires its input to be a deterministic context-free grammar, a subset of context-free grammars, that can be implemented using a deterministic pushdown automaton; restructering a context-free grammar into a deterministic context-free grammar is the part of using yacc that mangles the mind the most). While Bison supports GLR parsing, this only extends its input to all context-free grammars; the same notation is used to specify grammars and this does not support the specification of languages such as: , where and denotes repeated times, which requires a context-free grammar. So the yacc grammar notation is not capable of expressing all computations.
Yorick is designed for processing large amounts of scientific data and is a lot like R, with slightly different syntax. Use of Yorick seems to have died out in the last few years. What happened, did its users discover R? I would certainly recommend a switch to R, but I would also suggest Yorick as a subject of study for the maintainers of R; there are language ideas to be mulled over, e.g.,
global("blah") to declare
blah as being a global variable within the function containing the call.
Things to read
The Theory of Parsing, Translation, and Compiling, Vol. 1, Parsing by Alfred V. Aho and Jeffrey D. Ullman. Warning, reading this book will make your head hurt.
Introduction to Automata Theory, Languages, and Computation by John Hopcroft and Jeffrey D. Ullman. Has a great chapter on Turing machines. If you really want your head to hurt, then get the book above rather than the first edition of this book.