feat(#77): implement multimiplication compilation

src/main/java/org/eolang/opeo/ast/Mul.java

@@ -23,6 +23,7 @@
  */
 package org.eolang.opeo.ast;
 
+import java.util.ArrayList;
 import java.util.List;
 import org.xembly.Directive;
 import org.xembly.Directives;
@@ -69,6 +70,9 @@ public Iterable<Directive> toXmir() {
 
     @Override
     public List<AstNode> opcodes() {
-        throw new UnsupportedOperationException("Not implemented yet");
+        final List<AstNode> res = new ArrayList<>(0);
+        res.addAll(this.left.opcodes());
+        res.addAll(this.right.opcodes());
+        return res;
     }
 }