Still doesn't compile, but I think excellent progress.

src/c/payloads/psse_string.c

@@ -20,11 +20,25 @@
 #include "memory/pso4.h"
 #include "memory/tags.h"
 
+#include "ops/string_ops.h"
 #include "payloads/cons.h"
 
 #include "ops/stack_ops.h"
 
 
+/**
+ * Construct a string from the character `c` and this `tail`. A string is
+ * implemented as a flat list of cells each of which has one character and a
+ * pointer to the next; in the last cell the pointer to next is NIL.
+ *
+ * @param c the character to add (prepend);
+ * @param tail the string which is being built.
+ */
+struct pso_pointer make_string( struct pso_pointer frame_pointer, wint_t c,
+                                struct pso_pointer tail ) {
+    return make_string_like_thing( frame_pointer, c, tail, STRINGTAG );
+}
+
 /**
  * @brief When an string is freed, its cdr pointer must be decremented.
  *