diff --git a/sources/container/qetcontainer.cpp b/sources/container/qetcontainer.cpp
index 19a7615b6..79050a8cd 100644
--- a/sources/container/qetcontainer.cpp
+++ b/sources/container/qetcontainer.cpp
@@ -19,6 +19,7 @@
 #include "qetcontainerdb.h"
 
 #include <QCryptographicHash>
+#include <functional>
 #include <QHash>
 #include <QMap>
 #include <QSet>
@@ -242,6 +243,28 @@ bool QetContainer::ownUuids(const QDomElement &project)
 	return true;
 }
 
+/**
+	@brief QetContainer::embeddedDefinitions
+	@return the paths of the symbol definitions the project @p project
+	holds in its collection, as a symbol's type names them after embed://
+*/
+QSet<QString> QetContainer::embeddedDefinitions(const QDomElement &project)
+{
+	QSet<QString> paths;
+	std::function<void(const QDomElement &, const QString &)> walk =
+			[&](const QDomElement &category, const QString &path) {
+		for (QDomElement child = category.firstChildElement() ; !child.isNull() ;
+			 child = child.nextSiblingElement()) {
+			const QString name = path + child.attribute(QStringLiteral("name"));
+			if (child.tagName() == QLatin1String("category")) walk(child, name + QLatin1Char('/'));
+			else if (child.tagName() == QLatin1String("element")) paths.insert(name);
+		}
+	};
+	for (const QDomElement &collection : childElements(project, QStringLiteral("collection")))
+		walk(collection, QString());
+	return paths;
+}
+
 /**
 	@brief QetContainer::dropComputed
 	Take out of the folio @p diagram what QElectroTech computes when it
@@ -253,19 +276,39 @@ bool QetContainer::ownUuids(const QDomElement &project)
 	list of its terminals. Checked on 342 projects: opened and saved, each
 	comes back as it was, but the terminal lists of a file whose symbols
 	share a uuid (F100), where wires find their ends by them.
+	Nothing is taken from a symbol whose definition is not among
+	@p defined (embeddedDefinitions()), nor from a wire ending on one:
+	QElectroTech cannot build that symbol, keeps it as read and saves it
+	back (F127), so nothing of it would be computed again.
 */
-void QetContainer::dropComputed(QDomElement diagram, bool terminal_lists)
+void QetContainer::dropComputed(QDomElement diagram, bool terminal_lists,
+								const QSet<QString> &defined)
 {
+	QSet<QString> undefined;
+	for (const QDomElement &list : childElements(diagram, QStringLiteral("elements")))
+		for (const QDomElement &element : childElements(list, QStringLiteral("element"))) {
+			const QString type = element.attribute(QStringLiteral("type"));
+			if (!type.startsWith(QLatin1String("embed://"))
+				|| !defined.contains(type.mid(int(qstrlen("embed://")))))
+				undefined.insert(element.attribute(QStringLiteral("uuid")));
+		}
 	for (QDomElement list : childElements(diagram, QStringLiteral("conductors")))
 		for (QDomElement wire : childElements(list, QStringLiteral("conductor")))
+		{
+			if (undefined.contains(wire.attribute(QStringLiteral("element1")))
+				|| undefined.contains(wire.attribute(QStringLiteral("element2"))))
+				continue;
 			for (const char *end : {"1", "2"})
 				for (const char *name : {"element%1_label", "element%1_name", "element%1_linked",
 										 "terminalname%1"})
 					wire.removeAttribute(QString::fromLatin1(name).arg(QLatin1String(end)));
+		}
 	for (QDomElement list : childElements(diagram, QStringLiteral("elements")))
 	{
 		for (QDomElement element : childElements(list, QStringLiteral("element")))
 		{
+			if (undefined.contains(element.attribute(QStringLiteral("uuid"))))
+				continue;
 			if (terminal_lists)
 				for (QDomElement terminals : childElements(element, QStringLiteral("terminals")))
 					removeElement(terminals);
@@ -341,8 +384,9 @@ QList<QetZip::Entry> QetContainer::split(const QDomDocument &project, const QStr
 	int folio = 0;
 		//Nothing QElectroTech computes is kept (format 4)
 	const bool own_uuids = ownUuids(root);
+	const QSet<QString> defined = embeddedDefinitions(root);
 	for (QDomElement diagram : childElements(root, QStringLiteral("diagram")))
-		dropComputed(diagram, own_uuids);
+		dropComputed(diagram, own_uuids, defined);
 
 		//The parts of the folios kept as they were, and their pictures
 	QMap<QString, QByteArray> pictures;   //name -> png, once each
diff --git a/sources/container/qetcontainer.h b/sources/container/qetcontainer.h
index bdfd37c55..c9f936317 100644
--- a/sources/container/qetcontainer.h
+++ b/sources/container/qetcontainer.h
@@ -89,7 +89,9 @@ namespace QetContainer
 	void inlinePicture(QDomElement image, const QByteArray &bytes);
 
 		/// Take what QElectroTech computes out of a folio (format 4)
-	void dropComputed(QDomElement diagram, bool terminal_lists);
+	void dropComputed(QDomElement diagram, bool terminal_lists, const QSet<QString> &defined);
+		/// The symbol definitions a project holds, by path after embed://
+	QSet<QString> embeddedDefinitions(const QDomElement &project);
 		/// true if every folio, symbol and wire has a uuid of its own
 	bool ownUuids(const QDomElement &project);
 		/// Parse @p xml as a .qet is read (QETProject::openFile)
diff --git a/tests/qttest/tst_qetcontainer.cpp b/tests/qttest/tst_qetcontainer.cpp
index 92a5b2c64..08cddd408 100644
--- a/tests/qttest/tst_qetcontainer.cpp
+++ b/tests/qttest/tst_qetcontainer.cpp
@@ -77,9 +77,10 @@ class tst_qetcontainer : public QObject
 	{
 		QDomDocument copy = document.cloneNode(true).toDocument();
 		const bool own_uuids = QetContainer::ownUuids(copy.documentElement());
+		const QSet<QString> defined = QetContainer::embeddedDefinitions(copy.documentElement());
 		for (QDomElement diagram = copy.documentElement().firstChildElement(QStringLiteral("diagram"));
 			 !diagram.isNull() ; diagram = diagram.nextSiblingElement(QStringLiteral("diagram")))
-			QetContainer::dropComputed(diagram, own_uuids);
+			QetContainer::dropComputed(diagram, own_uuids, defined);
 		return copy;
 	}