SymbolicReduction

This stage represents Normalization Stage1: Symbolic Reduction in Philip Wadler's Paper "A Practical Theory of Language Integrated Query", given in Figure 11. https://homepages.inf.ed.ac.uk/slindley/papers/practical-theory-of-linq.pdf

It represents foundational normalizations done to sequences that represents queries. In Wadler's paper, he characterizes them as for x in P ...`` whereas in Quill they are characterized as list comprehensions i.e.P.flatMap(x => ...)`.