diff --git a/README.markdown b/README.markdown
index 1d8a63a..c3c5f78 100644
--- a/README.markdown
+++ b/README.markdown
@@ -83,6 +83,13 @@ Along with routines, you get `if`, `repeat`, and `with` constructs which take
 blocks.  The `with` construct takes an instruction like `sei` and implicitly
 (and unavoidably) inserts the corresponding `cli` at the end of the block.
 
+Abstract interpretation extends to `if` blocks.  The two incoming contexts are
+merged, and any storage locations poisoned in either context are considered
+poisoned in the result context.
+
+(Same should apply for `repeat` and `with` and, really, many other cases
+which there just aren't enough test cases for yet.)
+
 For More Information
 --------------------
 
@@ -94,11 +101,6 @@ Ideas
 -----
 
 These aren't implemented yet:
-    
-*   Abstract interpretation must extend to `if`, `repeat`, and `with`
-    blocks.  The two incoming contexts must be merged, and any storage
-    locations updated differently or poisoned in either context, will be
-    considered poisoned in the result context.
 
 *   Inside a routine, an address may be declared with `temporary`.  This is like
     `static` in C, except the value at that address is not guaranteed to be
diff --git a/doc/Analyzing.markdown b/doc/Analyzing.markdown
index 7f9d2f7..c609775 100644
--- a/doc/Analyzing.markdown
+++ b/doc/Analyzing.markdown
@@ -127,3 +127,72 @@ Routines can name registers as outputs.
     = 
     = update_score ([A])
     =   A: UpdatedWith (Immediate 8)
+
+If a location is poisoned in either branch of an `if`, it is poisoned
+after the `if`.
+
+    | reserve byte score
+    | routine update_score
+    | {
+    |   if beq {
+    |      lda #8
+    |   } else {
+    |      ldx #8
+    |   }
+    | }
+    | routine main {
+    |   lda #4
+    |   jsr update_score
+    |   sta score
+    | }
+    ? routine does not preserve 'A'
+
+    | reserve byte score
+    | routine update_score
+    | {
+    |   if beq {
+    |      ldx #8
+    |   } else {
+    |      lda #8
+    |   }
+    | }
+    | routine main {
+    |   lda #4
+    |   jsr update_score
+    |   sta score
+    | }
+    ? routine does not preserve 'A'
+
+    | reserve byte score
+    | routine update_score
+    | {
+    |   lda #4
+    |   sta score
+    | }
+    | routine main {
+    |   lda #4
+    |   if beq {
+    |      jsr update_score
+    |   } else {
+    |      ldx #3
+    |   }
+    |   sta score
+    | }
+    ? routine does not preserve 'A'
+
+    | reserve byte score
+    | routine update_score
+    | {
+    |   lda #4
+    |   sta score
+    | }
+    | routine main {
+    |   lda #4
+    |   if beq {
+    |      ldx #3
+    |   } else {
+    |      jsr update_score
+    |   }
+    |   sta score
+    | }
+    ? routine does not preserve 'A'
diff --git a/src/SixtyPical/Analyzer.hs b/src/SixtyPical/Analyzer.hs
index bc4a4da..a4e4539 100644
--- a/src/SixtyPical/Analyzer.hs
+++ b/src/SixtyPical/Analyzer.hs
@@ -73,10 +73,11 @@ analyzeProgram program@(Program decls routines) =
           -- TODO: mark Carry bit as "touched" here
           routCtx
       checkInstr (IF _ branch b1 b2) progCtx routCtx =
-          -- TODO: oooh, this one's gonna be fun
-          --checkBlock b1 progCtx routCtx
-          --checkBlock b2 progCtx routCtx
-          routCtx
+          let
+              routCtx1 = checkBlock b1 progCtx routCtx
+              routCtx2 = checkBlock b2 progCtx routCtx
+          in
+              mergeAlternateRoutCtxs routCtx1 routCtx2
       checkInstr (REPEAT _ branch blk) progCtx routCtx =
           -- TODO: oooh, this one's gonna be fun too
           --checkBlock blk progCtx routCtx
@@ -140,3 +141,29 @@ untypedLocation (NamedLocation (Just _) name) =
     NamedLocation Nothing name
 untypedLocation x = x
 
+--
+-- Utility function:
+-- Take 2 routine contexts -- one from each branch of an `if` -- and merge
+-- them to create a new context for the remainder of the routine.
+--
+mergeAlternateRoutCtxs routCtx1 routCtx2 =
+    let
+        -- go through all the Usages in routCtx2
+        -- insert any that were updated, into routCtx1
+        poison location usage2 routCtxAccum =
+            case Map.lookup location routCtx1 of
+                Nothing ->
+                    Map.insert location usage2 routCtxAccum
+                Just usage1 ->
+                    -- it exists in both routCtxs.
+                    -- if it is poisoned in either, it's poisoned here.
+                    -- otherwise, it is OK to differ.
+                    let
+                        newUsage = case (usage1, usage2) of
+                            (PoisonedWith _, _) -> usage1
+                            (_, PoisonedWith _) -> usage2
+                            _ -> usage1  -- or 2.  doesn't matter.
+                    in
+                        Map.insert location newUsage routCtxAccum
+    in
+        Map.foldrWithKey (poison) routCtx1 routCtx2