list(Return Values in SSA form): List of the return values in ssa form

def return_values_ssa(self):
        """
            list(Return Values in SSA form): List of the return values in ssa form
        """
        from slither.core.cfg.node import NodeType
        from slither.slithir.operations import Return
        from slither.slithir.variables import Constant

        if self._return_values_ssa is None:
            return_values_ssa = list()
            returns = [n for n in self.nodes if n.type == NodeType.RETURN]
            [return_values_ssa.extend(ir.values) for node in returns for ir in node.irs_ssa if isinstance(ir, Return)]
            self._return_values_ssa = list(set([x for x in return_values_ssa if not isinstance(x, Constant)]))
        return self._return_values_ssa