James H has implemented a new `Tricky' difficulty level in Light Up:

a non-recursive level above Easy, which therefore moves the
recursive Hard mode further up still. Play-testing suggests that in
fact Tricky is often _harder_ than the old Hard mode, since the
latter had limited depth of recursion and would therefore spot
complex deductions only if it happened to start a recursion on the
right square; Tricky may be limited in the sophistication of its
complex deductions, but it never misses one, so its puzzles tend to
be hard all over.

Also in this checkin, a new source file `nullfe.c', containing all
the annoying stub functions required to make command-line solvers
link successfully. James wrote this for (the new) lightupsolver, and
I've used it to simplify the other stand-alone solvers.

[originally from svn r6254]

puzzles.h

@@ -323,6 +323,20 @@ psdata *ps_init(FILE *outfile, int colour);
 void ps_free(psdata *ps);
 drawing *ps_drawing_api(psdata *ps);
 
+/*
+ * combi.c: provides a structure and functions for iterating over
+ * combinations (i.e. choosing r things out of n).
+ */
+typedef struct _combi_ctx {
+  int r, n, nleft, total;
+  int *a;
+} combi_ctx;
+
+combi_ctx *new_combi(int r, int n);
+void reset_combi(combi_ctx *combi);
+combi_ctx *next_combi(combi_ctx *combi); /* returns NULL for end */
+void free_combi(combi_ctx *combi);
+
 /*
  * Data structure containing the function calls and data specific
  * to a particular game. This is enclosed in a data structure so