This is a simple typedef of a specific type of OEUnaryFunction. The first
two template arguments indicate that an OEMolFunctionBase will have an
operator() that takes an OEChem::OEMolBase as an argument and will
return boolean value. The final two template arguments indicate that the
OEChem::OEMolBase argument will be const but that the operator()
function will not be const respectively. For more information on
OEUnaryFunctions, please see the OESystem documentation.