diff --git a/alib2algo/src/tree/generate/RandomTreeFactory.cpp b/alib2algo/src/tree/generate/RandomTreeFactory.cpp
index f618be9227b9c80a61580d880bcbebb183036e65..46a71bb57f96aae2bebec868850c25011872c48c 100644
--- a/alib2algo/src/tree/generate/RandomTreeFactory.cpp
+++ b/alib2algo/src/tree/generate/RandomTreeFactory.cpp
@@ -419,7 +419,7 @@ RankedExtendedPattern < > RandomRankedExtendedPatternFactory::generateRankedExte
 
 	ext::set < common::ranked_symbol < > > selectedNodeWildcards;
 	for ( const std::pair < const size_t, ext::vector < DefaultSymbolType > > & it : rankedAlphabet )
-		for ( DefaultSymbolType i : it.second )
+		for ( const DefaultSymbolType & i : it.second )
 			if ( ext::random_devices::semirandom ( ) % 100 < nodeWildcardProbability )
 				selectedNodeWildcards.insert ( common::ranked_symbol < > ( i, it.first ) );
 
@@ -450,7 +450,7 @@ RankedNonlinearPattern < > RandomRankedNonlinearPatternFactory::generateRankedNo
 	if ( singleNonlinearVariable )
 		nonlinearVariables.insert ( common::ranked_symbol < > ( DefaultSymbolType ( alphabet::NonlinearVariableSymbol < > ( DefaultSymbolType ( "A" ) ) ), 0 ) );
 	else
-		for ( DefaultSymbolType i : rankedAlphabet [ 0 ] )
+		for ( const DefaultSymbolType & i : rankedAlphabet [ 0 ] )
 			nonlinearVariables.insert ( common::ranked_symbol < > ( DefaultSymbolType ( alphabet::NonlinearVariableSymbol < > ( DefaultSymbolType ( i ) ) ), 0 ) );
 
 	common::ranked_symbol < > subtreeWildcard = alphabet::WildcardSymbol::instance < common::ranked_symbol < > > ( );