diff --git a/impurity/check_for_impurity.se b/impurity/check_for_impurity.se
new file mode 100644
index 0000000..e4ccae0
--- /dev/null
+++ b/impurity/check_for_impurity.se
@@ -0,0 +1,61 @@
+macro calldatachar($x):
+    div(calldataload($x), 2**248)
+
+# sum([2**x for x in [0x31, 0x32, 0x33, 0x3a, 0x3b, 0x3c, 0x40, 0x41, 0x42, 0x43, 0x44, 0x45, 0x54, 0x55, 0xf0, 0xff])
+mask = 57897811465722876096115075801844696845150819816717215668290421542284681019392
+
+data approved_addrs[]
+
+def submit(addr: address):
+    # Copy external contract code
+    extcode = string(~extcodesize(addr))
+    ~extcodecopy(addr, extcode, 0, ~extcodesize(addr))
+    ops = array(~extcodesize(addr))
+    pushargs = array(~extcodesize(addr))
+    # Loop through the code
+    with i = 0:
+        with op = 0:
+            while i < len(extcode):
+                with c = ~mod(~mload(extcode + i - 31), 256):
+                    # Banned opcode
+                    if ~and(2**c, mask):
+                        ~invalid()
+                    # PUSH
+                    if 0x60 <= c and c <= 0x7f:
+                        pushargs[op] = ~div(~mload(extcode + i + 1), 256 ** (0x7f - c))
+                        i += c - 0x5e
+                    # Call, callcode, delegatecall
+                    elif c == 0xf1 or c == 0xf2 or c == 0xf4:
+                        # Pattern-match two ways of setting the gas parameter:
+                        #
+                        # 1. PUSH<value>
+                        # 2. sub(gas, PUSH<value>)
+                        if op >= 2 and ops[op - 1] >= 0x60 and ops[op - 1] <= 0x7f:
+                            address_entry = op - 2
+                        elif op >= 4 and ops[op - 1] == 0x03 and ops[op - 2] == 0x5a and ops[op - 3] >= 0x60 and ops[op - 3] <= 0x7f:
+                            address_entry = op - 4
+                        else:
+                            ~invalid()
+                        # Operation before the gas parameter must satisfy one of three conditions:
+                        #
+                        # 1. It is a PUSH of an already approved address
+                        # 2. It is the address itself, through the ADDRESS opcode (ie. self-calling is permitted)
+                        # 3. It is a PUSH1, ie. less than 256 (ie. a present or future precompile)
+                        if self.approved_addrs[pushargs[address_entry]]:
+                            success = 1
+                        elif ops[address_entry] == 0x30:
+                            success = 1
+                        elif ops[address_entry] == 0x60:
+                            success = 1
+                        if not success:
+                            ~invalid()
+                        i += 1
+                    else:
+                        i += 1
+                    ops[op] = c
+                    op += 1
+    self.approved_addrs[addr] = 1
+    return(1: bool)
+
+def check(addr: address):
+    return(self.approved_addrs[addr]:bool)
diff --git a/impurity/impurity_tester.py b/impurity/impurity_tester.py
new file mode 100644
index 0000000..e1db22b
--- /dev/null
+++ b/impurity/impurity_tester.py
@@ -0,0 +1,122 @@
+from ethereum import tester as t
+from ethereum import utils
+from ethereum import transactions
+import rlp
+import serpent
+s = t.state()
+c = s.abi_contract('check_for_impurity.se')
+
+#from ethereum.slogging import LogRecorder, configure_logging, set_level
+#config_string = ':info,eth.vm.log:trace,eth.vm.op:trace,eth.vm.stack:trace,eth.vm.exit:trace,eth.pb.msg:trace,eth.pb.tx:debug'
+#configure_logging(config_string=config_string)
+
+test1 = s.abi_contract("""
+
+data horse
+
+def foo():
+    return self.horse
+
+""")
+
+try:
+    c.submit(test1.address)
+    success = True
+except:
+    success = False
+assert not success
+
+failedtest_addr = "0x"+utils.encode_hex(test1.address)
+
+test2 = s.abi_contract("""
+
+def foo():
+    return block.number
+
+""")
+
+try:
+    c.submit(test2.address)
+    success = True
+except:
+    success = False
+assert not success
+
+test3 = s.abi_contract("""
+
+def foo(x):
+    return x * 2
+""")
+
+c.submit(test3.address)
+
+
+
+test4 = s.abi_contract("""
+
+def modexp(b: uint256, e: uint256, m: uint256):
+    if e == 0:
+        return 1
+    elif e == 1:
+        return b
+    elif e % 2 == 0:
+        return self.modexp(~mulmod(b, b, m), ~div(e, 2), m)
+    elif e % 2 == 1:
+        return ~mulmod(self.modexp(~mulmod(b, b, m), ~div(e, 2), m), b, m)
+
+""")
+
+c.submit(test4.address)
+modexp_addr = "0x"+utils.encode_hex(test4.address)
+
+test5 = s.abi_contract("""
+
+def modinv(b, m):
+    inpdata = [0xa7d4bbe6, b, m-2, m]
+    outdata = [0]
+    ~call(100000, %s, 0, inpdata + 28, 100, outdata, 32)
+    return outdata[0]
+    
+""" % modexp_addr)
+
+c.submit(test5.address)
+
+test6 = s.abi_contract("""
+def phooey(h, v, r, s):
+    return ecrecover(h, v, r, s)
+""")
+
+c.submit(test6.address)
+
+test7 = s.abi_contract("""
+
+def modinv(b, m):
+    inpdata = [0xa7d4bbe6, b, m-2, m]
+    outdata = [0]
+    ~call(msg.gas - 10000, %s, 0, inpdata + 28, 100, outdata, 32)
+    return outdata[0]
+    
+""" % failedtest_addr)
+
+try:
+    c.submit(test7.address)
+    success = True
+except:
+    success = False
+assert not success
+
+print('All tests passed')
+
+kode = serpent.compile('check_for_impurity.se')
+
+# Create transaction
+t = transactions.Transaction(0, 30 * 10**9, 2999999, '', 0, kode)
+t.startgas = t.intrinsic_gas_used + 50000 + 200 * len(kode)
+t.v = 27
+t.r = 45
+t.s = 79
+print('Send %d wei to %s' % (t.startgas * t.gasprice,
+                             '0x'+utils.encode_hex(t.sender)))
+
+print('Contract address: 0x'+utils.encode_hex(utils.mk_contract_address(t.sender, 0)))
+print('Code: 0x'+utils.encode_hex(rlp.encode(t)))