fix tests; add comprehensive tests for functions and fix bugs in the mach VM regarding them.

mcode.cm

@@ -951,6 +951,19 @@ var mcode = function(ast) {
     return null
   }
 
+  // --- Helper: guard that reverse param is logical (or null) ---
+  // Disrupts if rev_slot is not null and not a boolean.
+  var emit_reverse_guard = function(rev_slot, msg) {
+    var ok_label = gen_label("rev_ok")
+    var g = alloc_slot()
+    emit_jump_cond("jump_null", rev_slot, ok_label)
+    emit_2("is_bool", g, rev_slot)
+    emit_jump_cond("jump_true", g, ok_label)
+    emit_log_error(msg)
+    emit_0("disrupt")
+    emit_label(ok_label)
+  }
+
   // --- Helper: forward loop scaffolding ---
   // L = {arr, len, i, check, item, one, loop_label, done_label}
   // body_fn(L) — called between element load and increment
@@ -1068,6 +1081,7 @@ var mcode = function(ast) {
     if (nargs <= 2) {
       emit_forward_loop(fwd_L, body_fn)
     } else {
+      emit_reverse_guard(args.rev, "arrfor: reverse must be a logical")
       emit_jump_cond("wary_true", args.rev, rev_label)
       emit_forward_loop(fwd_L, body_fn)
       emit_jump(final_label)
@@ -1289,6 +1303,9 @@ var mcode = function(ast) {
     emit_2("int", zero, 0)
     emit_2("int", one, 1)
     emit_1("null", null_s)
+    if (nargs > 2) {
+      emit_reverse_guard(args.rev, "find: reverse must be a logical")
+    }
     emit_2("is_func", is_fn, target)
     emit_jump_cond("jump_true", is_fn, fn_mode_label)
     // === Value mode ===
@@ -1523,6 +1540,7 @@ var mcode = function(ast) {
       d2 = gen_label("reduce_d2")
       d3 = gen_label("reduce_d3")
       d4 = gen_label("reduce_d4")
+      emit_reverse_guard(rev_slot, "reduce: reverse must be a logical")
       emit_2("is_null", check, init_slot)
       emit_jump_cond("jump_false", check, has_init)
       // No initial