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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 _displaystr2num(st):
 """Return a display number from a string"""

    num = None
    for s, n in [('DFP-', 16), ('TV-', 8), ('CRT-', 0)]:
        if st.startswith(s):
            try:
                curnum = int(st[len(s):])
                if 0 <= curnum <= 7:
                    num = n + curnum
                    break
            except Exception:
                pass
    if num is not None:
        return num
    else:
        raise ValueError('Unrecognised display name: ' + st)