fix drawing dfd with nested boundaries

[nineinchnick]

fix drawing dfd with nested boundaries

[ghost]

DeepCode's analysis on #402f79 found:

• :information_source: 12 minor issues. :point_down:

  Top issues

Description	Example fixes
Unnecessary parens after 'in' keyword Occurrences: pytm.py:575	:wrench: Example fixes
Comparing against a callable, did you omit the parenthesis? Occurrences: pytm.py:201 pytm.py:206 pytm.py:211	:wrench: Example fixes
Access to a protected member _safeset of a client class Occurrences: pytm.py:316 pytm.py:317 pytm.py:318	:wrench: Example fixes

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