example.rb

ary.sort do |left, right|
  IMPORTANT = %w(X Y)
  if IMPORTANT.include?(left) && IMPORTANT.include?(right)
    IMPORTANT.index(left) <=> IMPORTANT.index(right)
  elsif IMPORTANT.include?(left)
    1
  elsif IMPORTANT.include?(right)
    -1
  else
    left <=> right
  end
end