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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 cmd(self):
 """ The command below covers most cases, if you need someting more complex subclass this. """

        cmd = (
            [self.compiler_binary] +
            self.flags +
            ['-U'+x for x in self.undef] +
            ['-D'+x for x in self.define] +
            ['-I'+x for x in self.include_dirs] +
            self.sources
        )
        if self.run_linker:
            cmd += (['-L'+x for x in self.library_dirs] +
                    [(x if os.path.exists(x) else '-l'+x) for x in self.libraries] +
                    self.linkline)
        counted = []
        for envvar in re.findall('\$\{(\w+)\}', ' '.join(cmd)):
            if os.getenv(envvar) is None:
                if envvar not in counted:
                    counted.append(envvar)
                    msg = "Environment variable '{}' undefined.".format(
                        envvar)
                    self.logger.error(msg)
                    raise CompilationError(msg)
        return cmd