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diff --git a/subprojects/viatra-runtime/src/main/java/tools/refinery/viatra/runtime/matchers/util/IMemory.java b/subprojects/viatra-runtime/src/main/java/tools/refinery/viatra/runtime/matchers/util/IMemory.java
new file mode 100644
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--- /dev/null
+++ b/subprojects/viatra-runtime/src/main/java/tools/refinery/viatra/runtime/matchers/util/IMemory.java
@@ -0,0 +1,81 @@
+/*******************************************************************************
+ * Copyright (c) 2010-2017, Gabor Bergmann, IncQueryLabs Ltd.
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0 which is available at
+ * http://www.eclipse.org/legal/epl-v20.html.
+ * 
+ * SPDX-License-Identifier: EPL-2.0
+ *******************************************************************************/
+package tools.refinery.viatra.runtime.matchers.util;
+
+/**
+ * A memory containing a positive or negative number of equal() copies for some values.
+ * During iterations, each distinct value is iterated only once.
+ * 
+ * <p> Refined by: <ul> 
+ *  <li>{@link IMultiset}, which always contains values with a nonnegative multiplicity. </li>
+ *  <li>{@link IDeltaBag}, which may contain values with negative multiplicity. </li>
+ *  <li>{@link ISetMemory}, which is just a set (allowed multiplicities: 0 and 1). </li>
+ * </ul>
+ * 
+ * @author Gabor Bergmann
+ * @since 1.7
+ * @noimplement This interface is not intended to be implemented by clients.
+ */
+public interface IMemory<T> extends IMemoryView<T>, Clearable {
+
+    /**
+     * Adds one value occurrence to the memory.
+     * 
+     * @return true if the tuple was not present before in the memory, or
+     *      (in case of {@link IDeltaBag}) is no longer present in the memory 
+     */
+    boolean addOne(T value);
+
+    /**
+     * Adds the given number of occurrences to the memory. The count value may or may not be negative.
+     * <p> Precondition if {@link IMultiset}: at least the given amount of occurrences exist, if count is negative.
+     * <p> Precondition if {@link ISetMemory}: count is +1 or -1, the latter is only allowed if the set contains the value.
+     * 
+     * @param count
+     *            the number of occurrences
+     * @return true if the tuple was not present before in the memory, or is no longer present in the memory
+     * @throws IllegalStateException if {@link IMultiset} or {@link ISetMemory} and the number of occurrences in the memory would underflow to negative
+     */
+    boolean addSigned(T value, int count);
+
+    /**
+     * Removes one occurrence of the given value from the memory.
+     * <p> Precondition if {@link IMultiset} or {@link ISetMemory}: the value must have a positive amount of occurrences in the memory.
+     * 
+     * @return true if this was the the last occurrence of the value, or
+     *      (in case of {@link IDeltaBag}) is the first negative occurrence of the value 
+     * @throws IllegalStateException if {@link IMultiset} or {@link ISetMemory} and value had no occurrences in the memory
+     */
+    boolean removeOne(T value);
+    
+    /**
+     * Removes one occurrence of the given value from the memory, if possible.
+     * 
+     * <p> Memory is unchanged and false is returned if 
+     *   {@link IMultiset} or {@link ISetMemory} and value had no occurrences in the memory
+     * 
+     * @return true if this was the the last occurrence of the value, or
+     *      (in case of {@link IDeltaBag}) is the first negative occurrence of the value 
+     *      
+     * @since 2.3
+     */
+    boolean removeOneOrNop(T value);
+
+    /**
+     * Removes all occurrences of the given value from the memory.
+     */
+    void clearAllOf(T value);
+
+    /**
+     * Empties out the memory. 
+     */
+    @Override
+    void clear();
+
+}
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