l_sys.rb

#!/usr/bin/env ruby

class LSystem
  attr_accessor :axiom
  attr_accessor :rules

  def initialize(axiom, rules)
    @axiom = axiom
    @rules = rules
    @layer = [axiom]
  end

  def generate
    (0..5).each do || 
      previous_layer = @layer
      @layer = []
      previous_layer.each do |var|
        @layer << transform(var)
        @layer.flatten!
      end
      puts @layer.join(' ')
    end
  end

  def transform(var)
    @rules.each do |r|
      return r[:to] if r[:from] == var
    end
    var
  end

end
rules = [
  { from: 'a', to: ['b'] },
  { from: 'b', to: %w[a c] },
  { from: 'c', to: %w[b a] }
]

sys = LSystem.new('a', rules)
puts sys.generate