diff --git a/apps/advent2020/4a.mu b/apps/advent2020/4a.mu
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+# https://adventofcode.com/2020/day/4
+#
+# To run (on Linux):
+#   $ git clone https://github.com/akkartik/mu
+#   $ cd mu
+#   $ ./translate_mu apps/advent2020/4a.mu
+#   $ ./a.elf < input
+#
+# You'll need to register to download the 'input' file for yourself.
+
+fn main -> _/ebx: int {
+  var curr-passport-field-count/esi: int <- copy 0
+  var valid-passport-count/edi: int <- copy 0
+  var line-storage: (stream byte 0x100)  # 256 bytes
+  var line/ecx: (addr stream byte) <- address line-storage
+  var slice-storage: slice
+  var slice/edx: (addr slice) <- address slice-storage
+  $main:line-loop: {
+    # read line from stdin
+    clear-stream line
+    read-line-from-real-keyboard line
+    # if line is empty (not even a newline), quit
+    var done?/eax: boolean <- stream-empty? line
+    compare done?, 0  # false
+    break-if-!=
+    print-stream-to-real-screen line
+    # if line has just a newline, process passport
+    skip-chars-matching-whitespace line
+    var new-passport?/eax: boolean <- stream-empty? line
+    {
+      compare new-passport?, 0  # false
+      break-if-=
+      compare curr-passport-field-count, 7
+      {
+        break-if-!=
+        valid-passport-count <- increment
+        print-string 0, "=> "
+        print-int32-decimal 0, valid-passport-count
+        print-string 0, "\n"
+      }
+      curr-passport-field-count <- copy 0
+      loop $main:line-loop
+    }
+    $main:word-loop: {
+      next-word line, slice
+      var done?/eax: boolean <- slice-empty? slice
+      compare done?, 0  # false
+      break-if-!=
+      print-string 0, "  "
+      print-slice-to-real-screen slice
+      # treat cid as optional
+      var optional?/eax: boolean <- slice-starts-with? slice, "cid:"
+      compare optional?, 0  # false
+      {
+        break-if-!=
+        # otherwise assume there are no invalid fields and no duplicate fields
+        curr-passport-field-count <- increment
+        print-string 0, " => "
+        print-int32-decimal 0, curr-passport-field-count
+      }
+      print-string 0, "\n"
+      loop
+    }
+    loop
+  }
+  # process final passport
+  compare curr-passport-field-count, 7
+  {
+    break-if-!=
+    valid-passport-count <- increment
+  }
+  print-int32-decimal 0, valid-passport-count
+  print-string 0, "\n"
+  return 0
+}