Supplement argument consistency checking.

With the need to start handling the supplemental argument,
I'll need a hook for those handlers to slip in their checks
that nothing is wrong.

sc2/src/uqm/planets/generate-base.cpp

@@ -79,6 +79,8 @@ namespace Planets  ::detail::  Generate_m
 
 	Generator::~Generator()= default;
 
+	void Generator::checkSupplementRequirements( const std::optional< std::string > & ) {}
+
 
 	// All of the default C++ generator functions forward to the 
 	// plain C default impls.  The idea is that you derive your
@@ -273,6 +275,12 @@ namespace Planets  ::detail::  Generate_m
 
 					throw std::runtime_error{ mesg };
 				}
+
+				std::cerr << "Performing supplementary argument checks for `" << star.supplement
+						<< "`..." << std::endl;
+				const auto name= star.supplement;
+				auto gen= registry().at( name ).get();
+				gen->checkSupplementRequirements( star.supArg ? star.supArg : std::optional< std::string >{} );
 			}
 		}
 	}

sc2/src/uqm/planets/generate.h

@@ -123,6 +123,7 @@ void SetupGeneratorForSolarSystem( STAR_DESC *desc, SOLARSYS_STATE *state );
 
 #include <memory>
 #include <string>
+#include <optional>
 
 namespace Planets  ::detail::  Generate_m
 {
@@ -168,6 +169,8 @@ namespace Planets  ::detail::  Generate_m
 					PLANET_DESC *world, COUNT whichNode )= 0;
 			virtual bool pickupLife( SolarSystem *solarSys,
 					PLANET_DESC *world, COUNT whichNode )= 0;
+
+			virtual void checkSupplementRequirements( const std::optional< std::string > & ); // Normally does nothing.
 	};
 
 	class exports::DefaultGenerator