pyderasn.py | 17 ++++++++++-------

diff --git a/pyderasn.py b/pyderasn.py
index ef4146be4d705b169271ecc2c582231071981ed0cee66e360ac1f00617796fea..774968dee5b0485689133b9267c7d7015044dac34202aeb220aba7ebc6f0612c 100755
--- a/pyderasn.py
+++ b/pyderasn.py
@@ -2047,6 +2047,9 @@         for pp in self.pps_lenindef(decode_path):
             yield pp
 
 
+SET01 = frozenset(("0", "1"))
+
+
 class BitString(Obj):
     """``BIT STRING`` bit string type
 
@@ -2161,7 +2164,7 @@                     value.startswith("'")
             ):
                 if value.endswith("'B"):
                     value = value[1:-2]
-                    if not set(value) <= set(("0", "1")):
+                    if not frozenset(value) <= SET01:
                         raise ValueError("B's coding contains unacceptable chars")
                     return self._bits2octets(value)
                 elif value.endswith("'H"):
@@ -2189,7 +2192,7 @@                     raise ObjUnknown("BitString value: %s" % name)
                 bits.append(bit)
             if len(bits) == 0:
                 return self._bits2octets("")
-            bits = set(bits)
+            bits = frozenset(bits)
             return self._bits2octets("".join(
                 ("1" if bit in bits else "0")
                 for bit in six_xrange(max(bits) + 1)
@@ -3566,7 +3569,7 @@     @property
     def allowable_chars(self):
         if PY2:
             return self._allowable_chars
-        return set(six_unichr(c) for c in self._allowable_chars)
+        return frozenset(six_unichr(c) for c in self._allowable_chars)
 
 
 class NumericString(AllowableCharsMixin, CommonString):
@@ -3582,11 +3585,11 @@     __slots__ = ()
     tag_default = tag_encode(18)
     encoding = "ascii"
     asn1_type_name = "NumericString"
-    _allowable_chars = set(digits.encode("ascii") + b" ")
+    _allowable_chars = frozenset(digits.encode("ascii") + b" ")
 
     def _value_sanitize(self, value):
         value = super(NumericString, self)._value_sanitize(value)
-        if not set(value) <= self._allowable_chars:
+        if not frozenset(value) <= self._allowable_chars:
             raise DecodeError("non-numeric value")
         return value
 
@@ -3603,13 +3606,13 @@     __slots__ = ()
     tag_default = tag_encode(19)
     encoding = "ascii"
     asn1_type_name = "PrintableString"
-    _allowable_chars = set(
+    _allowable_chars = frozenset(
         (ascii_letters + digits + " '()+,-./:=?").encode("ascii")
     )
 
     def _value_sanitize(self, value):
         value = super(PrintableString, self)._value_sanitize(value)
-        if not set(value) <= self._allowable_chars:
+        if not frozenset(value) <= self._allowable_chars:
             raise DecodeError("non-printable value")
         return value