Write encoded XML parser and pretty-printer

Write a function that reads a string of compressed XML and outputs the
decompressed version.

Note to self: Now that I'm growing more comfortable writing parsers, I'd like to
become equally comfortable writing pretty-printers.

scratch/facebook/moderate/parser.py

@@ -0,0 +1,37 @@
+class Parser(object):
+    def __init__(self, tokens):
+        self.tokens = tokens
+        self.i = 0
+
+    def prev(self):
+        return self.tokens[self.i - 1]
+
+    def curr(self):
+        return self.tokens[self.i]
+
+    def next(self):
+        return self.tokens[self.i + 1]
+
+    def consume(self):
+        if not self.exhausted():
+            self.i += 1
+            return self.prev()
+
+    def match(self, xs):
+        if not self.exhausted() and self.curr() in xs:
+            self.consume()
+            return True
+        return False
+
+    def expect(self, xs):
+        if not self.match(xs):
+            raise Exception("Expected token \"{}\" but received \"{}\"".format(xs, self.curr()))
+        return self.prev()
+
+    def expect_predicate(self, predicate):
+        if predicate(self.curr()):
+            return self.consume()
+        raise Exception("Expected token \"{}\" to pass predicate, but it did not".format(self.curr()))
+
+    def exhausted(self):
+        return self.i >= len(self.tokens)