Rule 2 for the MIU System.

miu_system2.rb

# Returns true if we can apply Rule 2 to the given string.
# (Rule 2: If our string looks like Mx, we can change it to Mxx.)
def can_apply_rule2?(str)
  str.start_with?("M")
end

# Applies Rule 2: We replace the string Mx with Mxx.
def apply_rule2(str)
  str + str[1, str.length]
end