preprocessor: fix iterator types in ParsingDriver::svar()

ParsingDriver.cc

@@ -2004,7 +2004,8 @@ ParsingDriver::ms_variance_decomposition()
 void
 ParsingDriver::svar()
 {
-  OptionsList::num_options_t::const_iterator it0, it1, it2;
+  OptionsList::string_options_t::const_iterator it0, it1, it2;
+  OptionsList::num_options_t::const_iterator itn;
   OptionsList::vec_int_options_t::const_iterator itv;
 
   it0 = options_list.string_options.find("ms.coefficients");
@@ -2023,10 +2024,10 @@ ParsingDriver::svar()
           && it2 != options_list.string_options.end()))
     error("You may only pass one of 'coefficients', 'variances', or 'constants'.");
 
-  it0 = options_list.num_options.find("ms.chain");
-  if (it0 == options_list.num_options.end())
+  itn = options_list.num_options.find("ms.chain");
+  if (itn == options_list.num_options.end())
     error("A chain option must be passed to the svar statement.");
-  else if (atoi(it0->second.c_str()) <= 0)
+  else if (atoi(itn->second.c_str()) <= 0)
     error("The value passed to the chain option must be greater than zero.");
 
   itv = options_list.vector_int_options.find("ms.equations");