Create a top-level "third_party" directory

This directory now contains third_party software that is vendored into
the Servo source tree. The idea is that it would eventually hold
webrender and other crates from mozilla-central as well with a standard
patch management approach for each.

third_party/ply/example/BASIC/README

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+Inspired by a September 14, 2006 Salon article "Why Johnny Can't Code" by
+David Brin (http://www.salon.com/tech/feature/2006/09/14/basic/index.html),
+I thought that a fully working BASIC interpreter might be an interesting,
+if not questionable, PLY example.  Uh, okay, so maybe it's just a bad idea,
+but in any case, here it is.
+
+In this example, you'll find a rough implementation of 1964 Dartmouth BASIC
+as described in the manual at:
+
+   http://www.bitsavers.org/pdf/dartmouth/BASIC_Oct64.pdf
+
+See also:
+
+  http://en.wikipedia.org/wiki/Dartmouth_BASIC
+
+This dialect is downright primitive---there are no string variables
+and no facilities for interactive input. Moreover, subroutines and functions
+are brain-dead even more than they usually are for BASIC. Of course,
+the GOTO statement is provided.
+
+Nevertheless, there are a few interesting aspects of this example:
+
+  - It illustrates a fully working interpreter including lexing, parsing,
+    and interpretation of instructions.
+ 
+  - The parser shows how to catch and report various kinds of parsing
+    errors in a more graceful way.
+
+  - The example both parses files (supplied on command line) and
+    interactive input entered line by line.
+
+  - It shows how you might represent parsed information.  In this case,
+    each BASIC statement is encoded into a Python tuple containing the
+    statement type and parameters.  These tuples are then stored in
+    a dictionary indexed by program line numbers.
+
+  - Even though it's just BASIC, the parser contains more than 80
+    rules and 150 parsing states. Thus, it's a little more meaty than
+    the calculator example.
+
+To use the example, run it as follows:
+
+   % python basic.py hello.bas
+   HELLO WORLD
+   %
+
+or use it interactively:
+
+   % python basic.py
+   [BASIC] 10 PRINT "HELLO WORLD"
+   [BASIC] 20 END
+   [BASIC] RUN
+   HELLO WORLD
+   [BASIC]
+
+The following files are defined:
+
+   basic.py         - High level script that controls everything
+   basiclex.py      - BASIC tokenizer
+   basparse.py      - BASIC parser
+   basinterp.py     - BASIC interpreter that runs parsed programs.
+
+In addition, a number of sample BASIC programs (.bas suffix) are
+provided.  These were taken out of the Dartmouth manual.
+
+Disclaimer: I haven't spent a ton of time testing this and it's likely that
+I've skimped here and there on a few finer details (e.g., strictly enforcing
+variable naming rules).  However, the interpreter seems to be able to run
+the examples in the BASIC manual.
+
+Have fun!
+
+-Dave
+
+
+
+
+
+