A ruby snippet to recombine an array representing a stack of markup, outputting an orderly stack in parent/child relationship

stack_recombinations.rb

=begin
  <px:categories>
    <px:each>
      <px:category>
        <px:books>
          <px:each>
            <px:book>
              --- Get stack suggestions at this level
=end

stack = [nil, 'categories', 'each', 'category', 'books', 'each', 'book']
limit = stack.size
limit = 3 if stack.size > 3

combinations = 1.upto(limit).flat_map {|n|
  stack.compact.combination(n).to_a.reject!{ |a| a[0] == 'each' }
}.compact

pp combinations

[["categories"],
 ["category"],
 ["books"],
 ["book"],
 ["categories", "each"],
 ["categories", "category"],
 ["categories", "books"],
 ["categories", "each"],
 ["categories", "book"],
 ["category", "books"],
 ["category", "each"],
 ["category", "book"],
 ["books", "each"],
 ["books", "book"],
 ["categories", "each", "category"],
 ["categories", "each", "books"],
 ["categories", "each", "each"],
 ["categories", "each", "book"],
 ["categories", "category", "books"],
 ["categories", "category", "each"],
 ["categories", "category", "book"],
 ["categories", "books", "each"],
 ["categories", "books", "book"],
 ["categories", "each", "book"],
 ["category", "books", "each"],
 ["category", "books", "book"],
 ["category", "each", "book"],
 ["books", "each", "book"]]


pp combinations.collect{ |a| "px:#{a.join(':')}" }

["px:categories",
 "px:category",
 "px:books",
 "px:book",
 "px:categories:each",
 "px:categories:category",
 "px:categories:books",
 "px:categories:each",
 "px:categories:book",
 "px:category:books",
 "px:category:each",
 "px:category:book",
 "px:books:each",
 "px:books:book",
 "px:categories:each:category",
 "px:categories:each:books",
 "px:categories:each:each",
 "px:categories:each:book",
 "px:categories:category:books",
 "px:categories:category:each",
 "px:categories:category:book",
 "px:categories:books:each",
 "px:categories:books:book",
 "px:categories:each:book",
 "px:category:books:each",
 "px:category:books:book",
 "px:category:each:book",
 "px:books:each:book"]