A Non Degenerate Semi-Classical Lagrangian for Dilaton-Gravity in Two   Dimensions
  An action for two dimensional gravity conformally coupled to two dilaton-type fields is analysed. Classically, the theory has some exact solutions. These include configurations representing black holes. A semi-classical theory is obtained by assuming that these singular solutions are caused by the collapse of some matter fields. The semi-classical equations of motion reveal then that any generic solution must have a flat geometry. 
Noureddine Mohammedi