1 #ifndef STAN_LANG_AST_NIL_HPP 2 #define STAN_LANG_AST_NIL_HPP 3 4 namespace stan { 5 namespace lang { 6 7 /** 8 * The nil structure used as a placeholder for undefined or empty 9 * values in several structures. 10 */ 11 struct nil { 12 }; 13 14 } 15 } 16 #endif 17