code: def fixpointmethod(self, cfg_node):
          
          JOIN = self.join(cfg_node)
          # Assignment check
          if isinstance(cfg_node, AssignmentNode):
              arrow_result = JOIN
  
              # Reassignment check
              if cfg_node.left_hand_side not in cfg_node.right_hand_side_variables:
                  # Get previous assignments of cfg_node.left_hand_side and remove them from JOIN
                  arrow_result = self.arrow(JOIN, cfg_node.left_hand_side)
  
              arrow_result = arrow_result | self.lattice.el2bv[cfg_node]
              constraint_table[cfg_node] = arrow_result
          # Default case
          else:
              constraint_table[cfg_node] = JOIN
docs: The most important part of PyT, where we perform
          the variant of reaching definitions to find where sources reach.