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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, mode=os.F_OK | os.X_OK, path=None):
 """ Given cmd, check where it is on PATH. Loosely based on the version in python 3.3. """


    if os.path.dirname(cmd):
        if os.path.isfile(cmd) and os.access(cmd, mode):
            return cmd

    if path is None:
        path = os.environ.get('PATH', defpath)
    if not path:
        return None

    paths = path.split(pathsep)

    if sys.platform == 'win32':
        # oh boy
        if curdir not in paths:
            paths = [curdir] + paths

        # also need to check the fileexts...
        pathext = os.environ.get('PATHEXT', '').split(pathsep)

        if any(cmd.lower().endswith(ext.lower()) for ext in pathext):
            files = [cmd]
        else:
            files = [cmd + ext for ext in pathext]
    else:
        # sanity
        files = [cmd]

    seen = set()
    for p in paths:
        normpath = normcase(p)
        if normpath in seen:
            continue
        seen.add(normpath)
        for f in files:
            fn = os.path.join(p, f)
            if os.path.isfile(fn) and os.access(fn, mode):
                return fn

    return None