python-like pseudocode for marks that implement

gpl_example_pseudocode

# Implementation of a mark that precludes gpl license
class no_gpl: mark
  propagation = propagation_types.called_code or propagation_types.contained_code
  
  # Override mark.can_bind(). Return reason why binding is 
  # impossible, or None on success.
  def can_bind(site, attachment):
    if attachment == mark_types.explicit_affirmative:
      if site.type.scope >= scope_types.code_block:
        if site.get_call_tree().is_marked('gpl', mark_types.explicit_affirmative):
          return "Is not compatible with GPL'ed code: %s" % site.get_url()
      return "Can only bind to code blocks or broader constructs."


# Implementation of the mark for a gpl license
class gpl: mark
  propagation = propagation_types.contained_code
  
  def can_bind(site):
    if attachment == mark_types.explicit_affirmative:
      if site.type.scope < scope_types.code_block:
        return "Can only bind to code blocks or broader constructs."