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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 which(cmd):
 """ Returns full path to a executable. Args: cmd (str):
 Executable command to search for. Returns: (str) Full path to command. None if it is not found. Example:: full_path_to_python = which("python") """

    def is_exe(fp):
        return os.path.isfile(fp) and os.access(fp, os.X_OK)

    fpath, fname = os.path.split(cmd)
    if fpath:
        if is_exe(cmd):
            return cmd
    else:
        for path in os.environ["PATH"].split(os.pathsep):
            exe_file = os.path.join(path, cmd)
            if is_exe(exe_file):
                return exe_file
    return None