code: def parse_source_file(source_file):
      
      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
docs: Parses a source file thing and returns the file name
  
      Example:
  
      >>> parse_file('hg:hg.mozilla.org/releases/mozilla-esr52:js/src/jit/MIR.h:755067c14b06')
      '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