Lots more code written, and I think most of it's OK; but it doesn't compile yet.

src/c/debug.h

@@ -0,0 +1,96 @@
+/**
+ *  debug.h
+ *
+ *  Post Scarcity Software Environment: entry point.
+ *
+ *  Print debugging output.
+ *
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#ifndef __psse_debug_h
+#define __psse_debug_h
+#include <ctype.h>
+#include <stdbool.h>
+#include <stdio.h>
+
+/**
+ * @brief Print messages debugging memory allocation. 
+ *
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_ALLOC 1
+
+/**
+ * @brief Print messages debugging arithmetic operations.
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_ARITH 2
+
+/**
+ * @brief Print messages debugging symbol binding.
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_BIND 4
+
+/**
+ * @brief Print messages debugging bootstrapping and teardown.
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_BOOTSTRAP 8
+
+/**
+ * @brief Print messages debugging evaluation.
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_EVAL 16
+
+/**
+ * @brief Print messages debugging input/output operations.
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_IO 32
+
+/**
+ * @brief Print messages debugging lambda functions (interpretation).
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_LAMBDA 64
+
+/**
+ * @brief Print messages debugging the read eval print loop.
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_REPL 128
+
+/**
+ * @brief Print messages debugging stack operations.
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_STACK 256
+
+/**
+ * @brief Print messages about equality tests.
+ * 
+ * Flag interpretation for the value of `verbosity`, defined in `debug.c`, q.v.
+ */
+#define DEBUG_EQUAL 512
+
+/**
+ * @brief Verbosity (and content) of debugging output
+ *
+ * Interpreted as a sequence of topic-specific flags, see above.
+ */
+extern int verbosity;
+
+#endif