Many different proposals for socialist economic systems call for a type of mixed economy, where multiple forms of ownership over the means of production co-exist with one another. Public enterprise employee managed market economies, another form of market socialism in which publicly owned, employee-managed production units engage in free market exchange of goods and services with one another as well as with final consumers, e.g. mid twentieth century Yugoslavia, Two more theoretical models are Prabhat Ranjan Sarkar's Progressive Utilization Theory and Economic democracy. Public enterprise participatory planning, an economy featuring social ownership of the means of production with allocation based on an integration of decentralized democratic planning, e.g. stateless communism, libertarian socialism.