private List<MappingFactor> resolveFolding(ModelDeclaration model, ModelSymbol source, AstModelFolding folding) {
assert model != null;
assert source != null;
assert folding != null;
ModelDeclaration sourceModel = source.findDeclaration();
assert sourceModel != null;
List<MappingFactor> results = Lists.create();
for (AstPropertyFolding propertyFolding : folding.properties) {
PropertyDeclaration targetProperty = model.findPropertyDeclaration(propertyFolding.target.identifier);
PropertyMappingKind mapping = resolveAggregateFunction(propertyFolding.aggregator);