diff --git a/femtolisp/builtins.c b/femtolisp/builtins.c
index 3d457ca..1140fc1 100644
--- a/femtolisp/builtins.c
+++ b/femtolisp/builtins.c
@@ -127,7 +127,7 @@ static value_t fl_exit(value_t *args, u_int32_t nargs)
 static value_t fl_symbol(value_t *args, u_int32_t nargs)
 {
     argcount("symbol", nargs, 1);
-    if (!isstring(args[0]))
+    if (!fl_isstring(args[0]))
         type_error("symbol", "string", args[0]);
     return symbol(cvalue_data(args[0]));
 }
diff --git a/femtolisp/cvalues.c b/femtolisp/cvalues.c
index ba77688..91b71b8 100644
--- a/femtolisp/cvalues.c
+++ b/femtolisp/cvalues.c
@@ -220,7 +220,7 @@ value_t string_from_cstr(char *str)
     return string_from_cstrn(str, strlen(str));
 }
 
-int isstring(value_t v)
+int fl_isstring(value_t v)
 {
     return (iscvalue(v) && cv_isstr((cvalue_t*)ptr(v)));
 }
diff --git a/femtolisp/flisp.c b/femtolisp/flisp.c
index 22c1067..dbc856a 100644
--- a/femtolisp/flisp.c
+++ b/femtolisp/flisp.c
@@ -214,6 +214,7 @@ void bounds_error(char *fname, value_t arr, value_t ind)
 
 // safe cast operators --------------------------------------------------------
 
+#define isstring fl_isstring
 #define SAFECAST_OP(type,ctype,cnvt)                                          \
 ctype to##type(value_t v, char *fname)                                        \
 {                                                                             \
@@ -226,6 +227,7 @@ SAFECAST_OP(symbol,symbol_t*,ptr)
 SAFECAST_OP(fixnum,fixnum_t, numval)
 SAFECAST_OP(cvalue,cvalue_t*,ptr)
 SAFECAST_OP(string,char*,    cvalue_data)
+#undef isstring
 
 // symbol table ---------------------------------------------------------------
 
@@ -711,7 +713,7 @@ value_t fl_cons(value_t a, value_t b)
     return c;
 }
 
-int isnumber(value_t v)
+int fl_isnumber(value_t v)
 {
     if (isfixnum(v)) return 1;
     if (iscprim(v)) {
@@ -1235,7 +1237,7 @@ static value_t apply_cl(uint32_t nargs)
             Stack[SP-1] = (issymbol(Stack[SP-1]) ? FL_T : FL_F); NEXT_OP;
         OP(OP_NUMBERP)
             v = Stack[SP-1];
-            Stack[SP-1] = (isnumber(v) ? FL_T:FL_F); NEXT_OP;
+            Stack[SP-1] = (fl_isnumber(v) ? FL_T:FL_F); NEXT_OP;
         OP(OP_FIXNUMP)
             Stack[SP-1] = (isfixnum(Stack[SP-1]) ? FL_T : FL_F); NEXT_OP;
         OP(OP_BOUNDP)
@@ -1947,7 +1949,7 @@ static value_t fl_function(value_t *args, uint32_t nargs)
 {
     if (nargs < 2 || nargs > 4)
         argcount("function", nargs, 2);
-    if (!isstring(args[0]))
+    if (!fl_isstring(args[0]))
         type_error("function", "string", args[0]);
     if (!isvector(args[1]))
         type_error("function", "vector", args[1]);
diff --git a/femtolisp/flisp.h b/femtolisp/flisp.h
index dacc32b..7b5a2b5 100644
--- a/femtolisp/flisp.h
+++ b/femtolisp/flisp.h
@@ -280,9 +280,9 @@ value_t cvalue_string(size_t sz);
 value_t cvalue_static_cstring(const char *str);
 value_t string_from_cstr(char *str);
 value_t string_from_cstrn(char *str, size_t n);
-int isstring(value_t v);
-int isnumber(value_t v);
-int isiostream(value_t v);
+int fl_isstring(value_t v);
+int fl_isnumber(value_t v);
+int fl_isiostream(value_t v);
 value_t cvalue_compare(value_t a, value_t b);
 int numeric_compare(value_t a, value_t b, int eq, int eqnans, char *fname);
 
diff --git a/femtolisp/iostream.c b/femtolisp/iostream.c
index f8cd27a..14c011b 100644
--- a/femtolisp/iostream.c
+++ b/femtolisp/iostream.c
@@ -38,7 +38,7 @@ void relocate_iostream(value_t oldv, value_t newv)
 cvtable_t iostream_vtable = { print_iostream, relocate_iostream,
                               free_iostream, NULL };
 
-int isiostream(value_t v)
+int fl_isiostream(value_t v)
 {
     return iscvalue(v) && cv_class((cvalue_t*)ptr(v)) == iostreamtype;
 }
@@ -46,7 +46,7 @@ int isiostream(value_t v)
 value_t fl_iostreamp(value_t *args, uint32_t nargs)
 {
     argcount("iostream?", nargs, 1);
-    return isiostream(args[0]) ? FL_T : FL_F;
+    return fl_isiostream(args[0]) ? FL_T : FL_F;
 }
 
 value_t fl_eof_object(value_t *args, uint32_t nargs)
@@ -64,7 +64,7 @@ value_t fl_eof_objectp(value_t *args, uint32_t nargs)
 
 static ios_t *toiostream(value_t v, char *fname)
 {
-    if (!isiostream(v))
+    if (!fl_isiostream(v))
         type_error(fname, "iostream", v);
     return value2c(ios_t*, v);
 }
diff --git a/femtolisp/print.c b/femtolisp/print.c
index 82e1e2c..26b7100 100644
--- a/femtolisp/print.c
+++ b/femtolisp/print.c
@@ -166,7 +166,7 @@ static inline int tinyp(value_t v)
 {
     if (issymbol(v))
         return (u8_strwidth(symbol_name(v)) < SMALL_STR_LEN);
-    if (isstring(v))
+    if (fl_isstring(v))
         return (cv_len((cvalue_t*)ptr(v)) < SMALL_STR_LEN);
     return (isfixnum(v) || isbuiltin(v));
 }
@@ -174,7 +174,7 @@ static inline int tinyp(value_t v)
 static int smallp(value_t v)
 {
     if (tinyp(v)) return 1;
-    if (isnumber(v)) return 1;
+    if (fl_isnumber(v)) return 1;
     if (iscons(v)) {
         if (tinyp(car_(v)) && (tinyp(cdr_(v)) ||
                                (iscons(cdr_(v)) && tinyp(car_(cdr_(v))) &&
diff --git a/femtolisp/string.c b/femtolisp/string.c
index 465323c..3c518da 100644
--- a/femtolisp/string.c
+++ b/femtolisp/string.c
@@ -19,7 +19,7 @@
 value_t fl_stringp(value_t *args, u_int32_t nargs)
 {
     argcount("string?", nargs, 1);
-    return isstring(args[0]) ? FL_T : FL_F;
+    return fl_isstring(args[0]) ? FL_T : FL_F;
 }
 
 value_t fl_string_count(value_t *args, u_int32_t nargs)
@@ -27,7 +27,7 @@ value_t fl_string_count(value_t *args, u_int32_t nargs)
     size_t start = 0;
     if (nargs < 1 || nargs > 3)
         argcount("string.count", nargs, 1);
-    if (!isstring(args[0]))
+    if (!fl_isstring(args[0]))
         type_error("string.count", "string", args[0]);
     size_t len = cv_len((cvalue_t*)ptr(args[0]));
     size_t stop = len;
@@ -66,7 +66,7 @@ value_t fl_string_width(value_t *args, u_int32_t nargs)
 value_t fl_string_reverse(value_t *args, u_int32_t nargs)
 {
     argcount("string.reverse", nargs, 1);
-    if (!isstring(args[0]))
+    if (!fl_isstring(args[0]))
         type_error("string.reverse", "string", args[0]);
     size_t len = cv_len((cvalue_t*)ptr(args[0]));
     value_t ns = cvalue_string(len);
@@ -102,7 +102,7 @@ value_t fl_string_decode(value_t *args, u_int32_t nargs)
     else {
         argcount("string.decode", nargs, 1);
     }
-    if (!isstring(args[0]))
+    if (!fl_isstring(args[0]))
         type_error("string.decode", "string", args[0]);
     cvalue_t *cv = (cvalue_t*)ptr(args[0]);
     char *ptr = (char*)cv_data(cv);
@@ -123,7 +123,7 @@ extern value_t stream_to_string(value_t *ps);
 
 value_t fl_string(value_t *args, u_int32_t nargs)
 {
-    if (nargs == 1 && isstring(args[0]))
+    if (nargs == 1 && fl_isstring(args[0]))
         return args[0];
     value_t arg, buf = fl_buffer(NULL, 0);
     ios_t *s = value2c(ios_t*,buf);
@@ -276,7 +276,7 @@ value_t fl_string_find(value_t *args, u_int32_t nargs)
     else if (iscprim(v) && cp_class(cp) == bytetype) {
         return mem_find_byte(s, *(char*)cp_data(cp), start, len);
     }
-    else if (isstring(v)) {
+    else if (fl_isstring(v)) {
         cvalue_t *cv = (cvalue_t*)ptr(v);
         needlesz = cv_len(cv);
         needle = (char*)cv_data(cv);