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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def freeze(self, bytecode_compile=True):
 """Freeze the PEX. :param bytecode_compile: If True, precompile .py files into .pyc files when freezing code. Freezing the PEX writes all the necessary metadata and environment bootstrapping code. It may only be called once and renders the PEXBuilder immutable. """

    self._ensure_unfrozen('Freezing the environment')
    self._prepare_code_hash()
    self._prepare_manifest()
    self._prepare_bootstrap()
    self._prepare_main()
    if bytecode_compile:
      self._precompile_source()
    self._frozen = True