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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 parse_source_file(source_file):
 """Parses a source file thing and returns the file name Example: 'js/src/jit/MIR.h' :arg str source_file: the source file ("file") from a stack frame :returns: the filename or ``None`` if it couldn't determine one """

    if not source_file:
        return None

    vcsinfo = source_file.split(':')
    if len(vcsinfo) == 4:
        # These are repositories or cloud file systems (e.g. hg, git, s3)
        vcstype, root, vcs_source_file, revision = vcsinfo
        return vcs_source_file

    if len(vcsinfo) == 2:
        # These are directories on someone's Windows computer and vcstype is a
        # file system (e.g. "c:", "d:", "f:")
        vcstype, vcs_source_file = vcsinfo
        return vcs_source_file

    if source_file.startswith('/'):
        # These are directories on OSX or Linux
        return source_file

    # We have no idea what this is, so return None
    return None