code: def implicitly_declare_ro(instructions: List[AbstractInstruction]):
      
      ro_addrs: List[int] = []
      for instr in instructions:
          if isinstance(instr, Declare):
              # The user has declared their own memory
              # so they are responsible for all declarations and memory references.
              return instructions
  
          if isinstance(instr, Measurement):
              if instr.classical_reg is None:
                  continue
  
              if instr.classical_reg.name == 'ro':
                  ro_addrs += [instr.classical_reg.offset]
              else:
                  # The user has used a classical register named something other than "ro"
                  # so they are responsible for all declarations and memory references.
                  return instructions
  
      if len(ro_addrs) == 0:
          return instructions
  
      warnings.warn("Please DECLARE all memory. I'm adding a declaration for the `ro` register, "
                    "but I won't do this for you in the future.")
  
      new_instr = instructions.copy()
      new_instr.insert(0, Declare(name='ro', memory_type='BIT', memory_size=max(ro_addrs) + 1))
      return new_instr
docs: Implicitly declare a register named ``ro`` for backwards compatibility with Quil 1.
  
      There used to be one un-named hunk of classical memory. Now there are variables with
      declarations. Instead of::
  
          MEASURE 0 [0]
  
      You must now measure into a named register, idiomatically::
  
          MEASURE 0 ro[0]
  
      The ``MEASURE`` instruction will emit this (with a deprecation warning) if you're still
      using bare integers for classical addresses. However, you must also declare memory in the
      new scheme::
  
          DECLARE ro BIT[8]
          MEASURE 0 ro[0]
  
      This method will determine if you are in "backwards compatibility mode" and will declare
      a read-out ``ro`` register for you. If you program contains any DECLARE commands or if it
      does not have any MEASURE x ro[x], this will not do anything.
  
      This behavior is included for backwards compatibility and will be removed in future releases
      of PyQuil. Please DECLARE all memory including ``ro``.