diff --git a/kernel/pattern.h b/kernel/pattern.h
index 13892ebc5..bb794e9fa 100644
--- a/kernel/pattern.h
+++ b/kernel/pattern.h
@@ -19,8 +19,12 @@ YOSYS_NAMESPACE_BEGIN
  *  Used for analyzing MUX tree control paths in DFF optimization.
  */
 
-typedef std::map<RTLIL::SigBit, bool> pattern_t; // Control signal -> required vals
+// Pattern matching for clock enable
+// A pattern maps control signals to their required values for a MUX path
+typedef std::map<RTLIL::SigBit, bool> pattern_t; // Set of control signals that must ALL match required vals
 typedef std::set<pattern_t> patterns_t;          // Alternative patterns (OR)
+typedef std::pair<RTLIL::SigBit, bool> ctrl_t;   // Control signal
+typedef std::set<ctrl_t> ctrls_t;                // Set of control signals that must ALL be active
 
 /**
  * Find if two patterns differ in exactly one variable.
diff --git a/passes/opt/opt_dff.cc b/passes/opt/opt_dff.cc
index ff14d367f..90ace69e5 100644
--- a/passes/opt/opt_dff.cc
+++ b/passes/opt/opt_dff.cc
@@ -56,11 +56,6 @@ struct OptDffWorker
 	dict<SigBit, int> bitusers;       // Signal sink count
 	dict<SigBit, cell_int_t> bit2mux; // Signal bit to driving MUX
 
-	// Pattern matching for clock enable
-	typedef std::map<RTLIL::SigBit, bool> pattern_t;
-	typedef std::pair<RTLIL::SigBit, bool> ctrl_t;   // Control signal
-	typedef std::set<ctrl_t> ctrls_t;                // Control signals (AND)
-
 	std::vector<Cell *> dff_cells;
 
 	bool is_active(SigBit sig, bool pol) const {