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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 install_string(self):
 """ Add every missing file to the install string shown to the user in an error message. """

        args = [
            "--reference-name", self.reference_name,
            "--annotation-name", self.annotation_name]
        if self.annotation_version:
            args.extend(["--annotation-version", str(self.annotation_version)])
        if self.requires_gtf:
            args.append("--gtf")
            args.append("\"%s\"" % self._gtf_path_or_url)
        if self.requires_protein_fasta:
            args += [
                "--protein-fasta \"%s\"" %
                path for path in self._protein_fasta_paths_or_urls]
        if self.requires_transcript_fasta:
            args += [
                "--transcript-fasta \"%s\"" %
                path for path in self._transcript_fasta_paths_or_urls]
        return "pyensembl install %s" % " ".join(args)