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diff --git a/prism/static_literals.h b/prism/static_literals.h
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+/**
+ * @file static_literals.h
+ *
+ * A set of static literal nodes that can be checked for duplicates.
+ */
+#ifndef PRISM_STATIC_LITERALS_H
+#define PRISM_STATIC_LITERALS_H
+
+#include "prism/defines.h"
+#include "prism/ast.h"
+#include "prism/node.h"
+#include "prism/parser.h"
+
+#include <assert.h>
+#include <stdbool.h>
+
+/**
+ * Certain sets of nodes (hash keys and when clauses) check for duplicate nodes
+ * to alert the user of potential issues. To do this, we keep a set of the nodes
+ * that have been seen so far, and compare whenever we find a new node.
+ *
+ * We bucket the nodes based on their type to minimize the number of comparisons
+ * that need to be performed.
+ */
+typedef struct {
+    /**
+     * This is the set of IntegerNode and SourceLineNode instances. We store
+     * them in a sorted list so that we can binary search through them to find
+     * duplicates.
+     */
+    pm_node_list_t integer_nodes;
+
+    /**
+     * This is the set of FloatNode instances. We store them in a sorted list so
+     * that we can binary search through them to find duplicates.
+     */
+    pm_node_list_t float_nodes;
+
+    /**
+     * This is the set of RationalNode instances. We store them in a flat list
+     * that must be searched linearly.
+     */
+    pm_node_list_t rational_nodes;
+
+    /**
+     * This is the set of ImaginaryNode instances. We store them in a flat list
+     * that must be searched linearly.
+     */
+    pm_node_list_t imaginary_nodes;
+
+    /**
+     * This is the set of StringNode and SourceFileNode instances. We store them
+     * in a sorted list so that we can binary search through them to find
+     * duplicates.
+     */
+    pm_node_list_t string_nodes;
+
+    /**
+     * This is the set of RegularExpressionNode instances. We store them in a
+     * sorted list so that we can binary search through them to find duplicates.
+     */
+    pm_node_list_t regexp_nodes;
+
+    /**
+     * This is the set of SymbolNode instances. We store them in a sorted list
+     * so that we can binary search through them to find duplicates.
+     */
+    pm_node_list_t symbol_nodes;
+
+    /**
+     * A pointer to the last TrueNode instance that was inserted, or NULL.
+     */
+    pm_node_t *true_node;
+
+    /**
+     * A pointer to the last FalseNode instance that was inserted, or NULL.
+     */
+    pm_node_t *false_node;
+
+    /**
+     * A pointer to the last NilNode instance that was inserted, or NULL.
+     */
+    pm_node_t *nil_node;
+
+    /**
+     * A pointer to the last SourceEncodingNode instance that was inserted, or
+     * NULL.
+     */
+    pm_node_t *source_encoding_node;
+} pm_static_literals_t;
+
+/**
+ * Add a node to the set of static literals.
+ *
+ * @param parser The parser that created the node.
+ * @param literals The set of static literals to add the node to.
+ * @param node The node to add to the set.
+ * @return A pointer to the node that is being overwritten, if there is one.
+ */
+pm_node_t * pm_static_literals_add(const pm_parser_t *parser, pm_static_literals_t *literals, pm_node_t *node);
+
+/**
+ * Free the internal memory associated with the given static literals set.
+ *
+ * @param literals The set of static literals to free.
+ */
+void pm_static_literals_free(pm_static_literals_t *literals);
+
+#endif