diff --git a/cc_macro.c b/cc_macro.c
index 01240f8..1d7dfb4 100644
--- a/cc_macro.c
+++ b/cc_macro.c
@@ -18,6 +18,15 @@
 #include "gcc_req.h"
 
 void require(int bool, char* error);
+int numerate_string(char *a);
+
+struct conditional_inclusion {
+    struct conditional_inclusion* prev;
+    int include; /* 1 == include, 0 == skip */
+    int previous_condition_matched; /* 1 == all subsequent conditions treated as FALSE */
+};
+
+struct conditional_inclusion *conditional_inclusion_top;
 
 /* point where we are currently modifying the global_token list */
 struct token_list* macro_token;
@@ -56,18 +65,230 @@ void eat_newline_tokens()
     }
 }
 
-void macro_directive()
+int macro_expression();
+int macro_variable()
 {
-    /* unhandled macro directive; let's eat until a newline; om nom nom */
-    while (TRUE)
+    eat_current_token();
+    return 0;
+}
+int macro_number()
+{
+    int result = numerate_string(macro_token->s);
+    eat_current_token();
+    return result;
+}
+
+int macro_primary_expr()
+{
+    if ('-' == macro_token->s[0])
     {
         eat_current_token();
+        return -macro_primary_expr();
+    }
+    else if ('!' == macro_token->s[0])
+    {
+        eat_current_token();
+        return !macro_primary_expr();
+    }
+    else if ('(' == macro_token->s[0])
+    {
+        eat_current_token();
+        return macro_expression();
+    } else if (in_set(macro_token->s[0], "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_"))
+        return macro_variable();
+    else if (in_set(macro_token->s[0], "0123456789"))
+        return macro_number();
+    else
+        return 0; /* FIXME: error handling */
+}
 
-        if (NULL == macro_token)
-            return;
+int macro_additive_expr()
+{
+    int lhs = macro_primary_expr();
 
-        if ('\n' == macro_token->s[0])
-            return;
+    if (match("+", macro_token->s))
+    {
+        eat_current_token();
+        return lhs + macro_additive_expr();
+    }
+    else if (match("-", macro_token->s))
+    {
+        eat_current_token();
+        return lhs - macro_additive_expr();
+    }
+    else if (match("*", macro_token->s))
+    {
+        eat_current_token();
+        return lhs * macro_additive_expr();
+    }
+    else if (match("/", macro_token->s))
+    {
+        eat_current_token();
+        return lhs / macro_additive_expr();
+    }
+    else if (match("%", macro_token->s))
+    {
+        eat_current_token();
+        return lhs % macro_additive_expr();
+    }
+    else if (match(">>", macro_token->s))
+    {
+        eat_current_token();
+        return lhs >> macro_additive_expr();
+    }
+    else if (match("<<", macro_token->s))
+    {
+        eat_current_token();
+        return lhs << macro_additive_expr();
+    }
+    else
+    {
+        return lhs;
+    }
+}
+
+int macro_relational_expr()
+{
+    int lhs = macro_additive_expr();
+
+    if (match("<", macro_token->s))
+    {
+        eat_current_token();
+        return lhs < macro_relational_expr();
+    }
+    else if (match("<=", macro_token->s))
+    {
+        eat_current_token();
+        return lhs <= macro_relational_expr();
+    }
+    else if (match(">=", macro_token->s))
+    {
+        eat_current_token();
+        return lhs >= macro_relational_expr();
+    }
+    else if (match(">", macro_token->s))
+    {
+        eat_current_token();
+        return lhs > macro_relational_expr();
+    }
+    else if (match("==", macro_token->s))
+    {
+        eat_current_token();
+        return lhs == macro_relational_expr();
+    }
+    else if (match("!=", macro_token->s))
+    {
+        eat_current_token();
+        return lhs != macro_relational_expr();
+    }
+    else
+    {
+        return lhs;
+    }
+}
+
+int macro_bitwise_expr()
+{
+    int rhs;
+    int lhs = macro_relational_expr();
+
+    if (match("&", macro_token->s))
+    {
+        eat_current_token();
+        return lhs & macro_bitwise_expr();
+    }
+    else if (match("&&", macro_token->s))
+    {
+        eat_current_token();
+        rhs = macro_bitwise_expr();
+        return lhs && rhs;
+    }
+    else if (match("|", macro_token->s))
+    {
+        eat_current_token();
+        rhs = macro_bitwise_expr();
+        return lhs | rhs;
+    }
+    else if (match("||", macro_token->s))
+    {
+        eat_current_token();
+        rhs = macro_bitwise_expr();
+        return lhs || rhs;
+    }
+    else if (match("^", macro_token->s))
+    {
+        eat_current_token();
+        rhs = macro_bitwise_expr();
+        return lhs ^ rhs;
+    }
+    else
+    {
+        return lhs;
+    }
+}
+
+int macro_expression()
+{
+    return macro_bitwise_expr();
+}
+
+
+
+void macro_directive()
+{
+    struct conditional_inclusion *t;
+    int result;
+    /* FIXME: whitespace is allowed between "#"" and "if" */
+    if(match("#if", macro_token->s))
+    {
+        eat_current_token();
+        /* evaluate constant integer expression */
+        result = macro_expression();
+        /* push conditional inclusion */
+        t = calloc(1, sizeof(struct conditional_inclusion));
+        t->prev = conditional_inclusion_top;
+        conditional_inclusion_top = t;
+        t->include = TRUE;
+        if (FALSE == result)
+        {
+            t->include = FALSE;
+        }
+        t->previous_condition_matched = t->include;
+    }
+    else if (match("#elif", macro_token->s))
+    {
+        eat_current_token();
+        result = macro_expression();
+        conditional_inclusion_top->include = result && !conditional_inclusion_top->previous_condition_matched;
+        conditional_inclusion_top->previous_condition_matched =
+            conditional_inclusion_top->previous_condition_matched || conditional_inclusion_top->include;
+    }
+    else if (match("#else", macro_token->s))
+    {
+        eat_current_token();
+        conditional_inclusion_top->include = !conditional_inclusion_top->previous_condition_matched;
+    }
+    else if (match("#endif", macro_token->s))
+    {
+        eat_current_token();
+        /* pop conditional inclusion */
+        t = conditional_inclusion_top;
+        conditional_inclusion_top = conditional_inclusion_top->prev;
+        free(t);
+    }
+    else
+    {
+        /* unhandled macro directive; let's eat until a newline; om nom nom */
+        while (TRUE)
+        {
+            if (NULL == macro_token)
+                return;
+
+            if ('\n' == macro_token->s[0])
+                return;
+
+            eat_current_token();
+        }
     }
 }
 
@@ -93,7 +314,19 @@ void preprocess()
         else
         {
             start_of_line = FALSE;
-            macro_token = macro_token->next;
+            if (NULL == conditional_inclusion_top)
+            {
+                macro_token = macro_token->next;
+            }
+            else if (!conditional_inclusion_top->include)
+            {
+                /* rewrite the token stream to exclude the current token */
+                eat_current_token();
+            }
+            else
+            {
+                macro_token = macro_token->next;
+            }
         }
     }
 }
diff --git a/test/test.answers b/test/test.answers
index 5de1a78..228be68 100644
--- a/test/test.answers
+++ b/test/test.answers
@@ -169,8 +169,8 @@ a2a83f42119e646b389b98647cf6cf2aa9597185997c9453db746178c8c4c0bf  test/results/t
 698853b79efb30865a663c4863c050639eb21c7400008f7840830503928973d4  test/results/test0106-knight-native-binary
 45c2ba61dc209d7ffa39de9ff0f0a7f8f3ea4d7e38598c72f982fcaf9a05c84a  test/results/test0106-knight-posix-binary
 944580ff4aae38aafac139faf6eed5bfe4ff68b01a7a3adfa346de8803101182  test/results/test0106-x86-binary
-206905afbcc1ef41d5b241fb72381c0854792fedbbd467ea33c52728cc8946b6  test/results/test1000-aarch64-binary
-85a238be594b0bbd3d2613872cc15315f5ad79f4491c424f006f1e98dc6234ef  test/results/test1000-amd64-binary
-27bf47331a04a42cae324da6862f21625c093ee27e9cf2504b1c6c80b98e814a  test/results/test1000-armv7l-binary
-e5102501c6939447a87084fd3853d55dcc32b80142efaa72b5f9305d80026b84  test/results/test1000-knight-posix-binary
-c0c649496e3c5738667d3bef74a917253471e192b3e550152c083d1ccf579d09  test/results/test1000-x86-binary
+5d0a41c3ec339d6413167ab9e87d8d3444863d1dc12883592ad23669654cb7ce  test/results/test1000-aarch64-binary
+37952a1f53e7859408bee89a5521cb3ef818363fa4bda682a3696c24fe7ff645  test/results/test1000-amd64-binary
+8cf345632079de545b1fac3158c7a2d048fa64ec0fed927c4b1bbffabdef5b3f  test/results/test1000-armv7l-binary
+f761471e800c86dcbaa1be532a7a0ccd3f6100e76852891331adb171c64bc932  test/results/test1000-knight-posix-binary
+f5662f155218f9e73d92cfa28bab8d12c6ed407a5662cf583299ac4f5ec71f95  test/results/test1000-x86-binary
diff --git a/test/test1000/proof.answer b/test/test1000/proof.answer
index c138c20..7d87551 100644
--- a/test/test1000/proof.answer
+++ b/test/test1000/proof.answer
@@ -1 +1 @@
-21f64ec000afd8b05c10532c51806c04eee2120ece05141fa7c0c6fab5e56869  test/test1000/proof
+cb2ed985d814d493cd376d89ef5471d66cfd76d78a7bb22fffbb076df44e0aa9  test/test1000/proof