diff --git a/src/completion.hpp b/src/completion.hpp
new file mode 100644
index 0000000..a95dafc
--- /dev/null
+++ b/src/completion.hpp
@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2020 CESNET, https://photonics.cesnet.cz/
+ *
+ * Written by Václav Kubernát <kubernat@cesnet.cz>
+ *
+*/
+#include <set>
+#include <string>
+struct Completion {
+    enum class WhenToAdd {
+        Always,
+        IfFullMatch
+    };
+    bool operator<(const Completion& b) const;
+    bool operator==(const Completion& b) const;
+    std::string m_value;
+
+    /** A completion can have a suffix specified. This suffix is appended to the completion, if it's the only valid completion present in
+     * the ParserContext. For example, let's say there are two valid schema paths: "user/name", "user/nationality" and "user/city" and the
+     * user tries to complete this path: "user/n". The parser determines these completions: "name", "nationality", "city". The parser
+     * filters out "city", because it doesn't start with an "n", so valid completions are "name" and "nationality". Their common prefix is
+     * "na", so the input becomes "user/na". Next, the user changes the output to "user/natio", the completions will again be "name",
+     * "nationality", "city", but now, after filtering, only one single completion remains - "nationality". This is where m_suffix gets in
+     * play: since there is only one completion remaining, m_suffix can get added depending on the value of m_whenToAdd. It it's set to
+     * Always, the suffix gets appended always. That means, the actual completion would be "nationality" plus whatever m_suffix is set to.
+     * Otherwise (if m_whenToAdd is set to IfFullMatch), the suffix will only get added after the user input fully matches the completion.
+     * For example, if the user input is "user/natio", the completion becomes just "nationality", but if the user input is "user/nationality",
+     * the completion becomes "nationality" plus the suffix.
+     */
+    std::string m_suffix = "";
+    WhenToAdd m_whenToAdd = WhenToAdd::Always;
+};
+
+/** Returns a subset of the original set with only the strings starting with prefix */
+std::set<Completion> filterByPrefix(const std::set<Completion>& set, const std::string_view prefix);