code: def add_comes_from(self, basic_block):
          
          if basic_block is None:
              return
  
          if self.lock:
              return
  
          # Return if already added
          if basic_block in self.comes_from:
              return
  
          self.lock = True
          self.comes_from.add(basic_block)
          basic_block.add_goes_to(self)
          self.lock = False
docs: This simulates a set. Adds the basic_block to the comes_from
          list if not done already.