gistfile1.rb

class Object
  def ary?; is_a? Array; end
  def str?; is_a? String; end

  def nonempty_ary?
    ary? && (not empty?)
  end
end

def sexp_select sexp, symbols
  (1..(max_nesting_level = 20)).map {|n| sexp.flatten(n)
                                             .select(&:nonempty_ary?)
                                             .select {|e| symbols.include? e[0] }}
                               .select(&:nonempty_ary?)
                               .flatten(1)
end