Day 1. Assembly code is being generated, on a monkey see, monkey do basis;

but it doesn't link because of a missing library; and having moved source
files around the Makefile currently doesn't work.

src/c/error.h

@@ -0,0 +1,45 @@
+/**
+ *  error.h
+ *
+ *  Grendel: a compiling Beowulf reimplementation.
+ *
+ *  The error handling subsystem.
+ *
+ *  Error codes are detailed in section 6.3, page 32, of the
+ *  Lisp 1.5 Programmer's Manual. I shall in so far as possible follow
+ *  those codes.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#ifndef __grendel_error_h
+#define __grendel_error_h
+
+#include <stdint.h>
+
+uint32_t make_error( char* code);
+
+/**
+ * @brief error codes
+ *
+ *  Error codes are detailed in section 6.3, page 32, of the
+ *  Lisp 1.5 Programmer's Manual. I shall in so far as possible follow
+ *  those codes.
+ *
+ *  Error codes comprise a one or two alphabetic character category, followed
+ *  by a single digit number. A space is shown in the manual between the
+ *  category and the number, but I have not followed that here for reasons.
+ */
+
+#define APPLIED_FUNCTION_CALLED_ERROR 	(make_error( "A1"))
+#define UNDEFINED_FUNCTION_APPLY_ERROR	(make_error( "A2"))
+#define COND_UNSATISFIED_ERROR			(make_error( "A3"))
+#define SETQ_ON_NONEXISTANT_VAR_ERROR	(make_error( "A4"))
+#define SET_ON_NONEXISTANT_VAR_ERROR	(make_error( "A5"))
+#define NO_TARGET_FOR_GO_ERROR			(make_error( "A6"))
+#define TOO_MANY_ARGUMENTS_ERROR		(make_error( "A7"))
+#define UNBOUND_VARIABLE_ERROR			(make_error( "A8"))
+#define UNDEFINED_FUNCTION_EVAL_ERROR	(make_error( "A9"))
+
+#endif