diff --git a/crypto/func/auto-tests/tests/invalid-ident-1.fc b/crypto/func/auto-tests/tests/invalid-ident-1.fc
new file mode 100644
index 000000000..32d5406af
--- /dev/null
+++ b/crypto/func/auto-tests/tests/invalid-ident-1.fc
@@ -0,0 +1,15 @@
+;; this is allowed, lexer doesn't stop on it
+int get_false!() { return false; }
+
+_ main() {
+    var x = false;
+    if (!x) {
+        return 1;
+    }
+    return 0;
+}
+
+{-
+@compilation_should_fail
+@stderr '!' is reserved for the future
+-}
diff --git a/crypto/func/auto-tests/tests/invalid-ident-2.fc b/crypto/func/auto-tests/tests/invalid-ident-2.fc
new file mode 100644
index 000000000..c42727f7f
--- /dev/null
+++ b/crypto/func/auto-tests/tests/invalid-ident-2.fc
@@ -0,0 +1,11 @@
+_ main() {
+    var x = false;
+    var b = ! x;
+    return b;
+}
+
+{-
+@compilation_should_fail
+@stderr var b = ! x;
+@stderr '!' is reserved for the future
+-}
diff --git a/crypto/func/auto-tests/tests/s1.fc b/crypto/func/auto-tests/tests/s1.fc
index 0dce5d979..f320df9f2 100644
--- a/crypto/func/auto-tests/tests/s1.fc
+++ b/crypto/func/auto-tests/tests/s1.fc
@@ -1,4 +1,7 @@
 get slice ascii_slice() {
+  if (false) {
+    return "!string!"; // !string!
+  }
   return "string";
 }
 
diff --git a/crypto/func/auto-tests/tests/test-math.fc b/crypto/func/auto-tests/tests/test-math.fc
index 27fea3612..67d74fde9 100644
--- a/crypto/func/auto-tests/tests/test-math.fc
+++ b/crypto/func/auto-tests/tests/test-math.fc
@@ -4,7 +4,7 @@ forall X -> (tuple, ()) ~tset(tuple t, int idx, X val) pure asm(t val idx) "SETI
 
 ;; computes 1-acos(x)/Pi by a very simple, extremely slow (~70k gas) and imprecise method
 ;; fixed256 acos_prepare_slow(fixed255 x);
-int acos_prepare_slow_f255(int x) inline {
+int acos_prepare_slow_f255!(int x) inline {
   x -= (x == 0);
   int t = 1;
   repeat (255) {
@@ -16,14 +16,14 @@ int acos_prepare_slow_f255(int x) inline {
 
 ;; extremely slow (~70k gas) and somewhat imprecise (very imprecise when x is small), for testing only
 ;; fixed254 acos_slow(fixed255 x);
-int acos_slow_f255(int x) inline_ref {
-  int t = acos_prepare_slow_f255(x);
+int acos_slow_f255@(int x) inline_ref {
+  int t = acos_prepare_slow_f255!(x);
   return - mulrshiftr256(t + (-1 << 256), Pi_const_f254());
 }
 
 ;; fixed255 asin_slow(fixed255 x);
-int asin_slow_f255(int x) inline_ref {
-  int t = acos_prepare_slow_f255(abs(x)) % (1 << 255);
+int asin_slow_f255%(int x) inline_ref {
+  int t = acos_prepare_slow_f255!(abs(x)) % (1 << 255);
   return muldivr(t, Pi_const_f254(), 1 << 255) * sgn(x);
 }
 
@@ -116,13 +116,13 @@ int asin_f255_test(x) method_id(10023) {
     return asin_f255(x);
 }
 int asin_slow_f255_test(x) method_id(10024) {
-    return asin_slow_f255(x);
+    return asin_slow_f255%(x);
 }
 int acos_f255_test(x) method_id(10025) {
     return acos_f255(x);
 }
 int acos_slow_f255_test(x) method_id(10026) {
-    return acos_slow_f255(x);
+    return acos_slow_f255@(x);
 }
 int  fixed248::atan_test(x) method_id(10027) {
     return  fixed248::atan(x);
diff --git a/crypto/parser/lexer.cpp b/crypto/parser/lexer.cpp
index 418860ebe..db19911f1 100644
--- a/crypto/parser/lexer.cpp
+++ b/crypto/parser/lexer.cpp
@@ -311,12 +311,16 @@ const Lexem& Lexer::next() {
     return lexem;
   }
   int len = 0, pc = -0x100;
+  bool starts_with_excl = false;
   while (end < src.get_end_ptr()) {
     c = *end;
     bool repeated = (c == pc && is_repeatable(c));
     if (c == ' ' || c == 9 || (len && is_left_active(c) && !repeated)) {
       break;
     }
+    if (c == '!' && !len) {
+      starts_with_excl = true;
+    }
     ++len;
     ++end;
     if (is_right_active(c) && !repeated) {
@@ -325,6 +329,9 @@ const Lexem& Lexer::next() {
     pc = c;
   }
   lexem.set(std::string{src.get_ptr(), end}, src.here());
+  if (starts_with_excl && is_FunC() && lexem.str != "!=") {
+    throw ParseError(lexem.loc, "'!' is reserved for the future, don't use it in FunC code");
+  }
   src.set_ptr(end);
   // std::cerr << lexem.name_str() << ' ' << lexem.str << std::endl;
   return lexem;
diff --git a/crypto/parser/lexer.h b/crypto/parser/lexer.h
index 904d8b31d..38f163240 100644
--- a/crypto/parser/lexer.h
+++ b/crypto/parser/lexer.h
@@ -120,6 +120,11 @@ class Lexer {
  private:
   void set_spec(std::array<int, 3>& arr, std::string setup);
   bool is_multiline_quote(const char* begin, const char* end);
+
+  // this class (like all sources in /ton/crypto/parser) is shared between FunC and tlbc
+  // (in the future, I'll implement lexer from scratch, FunC/TL lexers would have nothing in common)
+  bool is_FunC() const { return cmt_op2[1] == 47; }
+  bool is_TLB() const { return !is_FunC(); }
 };
 
 }  // namespace src