for many platforms.) Other implementations PyPy is a fast, compliant interpreter of Python 2.7 and 3.8. Its just-in-time compiler often brings a significant speed improvement over CPython but some libraries written in C cannot be used with it. Stackless Python is a significant fork of CPython that implements microthreads; it does not use the call stack in the same way, thus allowing massively concurrent programs. PyPy also has a stackless version. MicroPython and CircuitPython are Python 3 variants optimized for microcontrollers, including Lego Mindstorms EV3. Pyston is a variant of the Python runtime that uses just-in-time compilation to speed up the execution of Python programs. Cinder is a performance-oriented fork of CPython 3.8 that contains a number of optimizations including bytecode inline caching, eager evaluation of coroutines, a method-at-a-time JIT, and an experimental bytecode compiler. Unsupported implementations Other just-in-time Python compilers have been developed, but are now unsupported: Google began a project named Unladen Swallow in 2009, with the aim of speeding up the Python interpreter fivefold by using the LLVM, and of improving its multithreading ability to scale to thousands of cores, while ordinary implementations suffer from the global interpreter lock. Psyco is a discontinued just-in-time specializing compiler that integrates with CPython and transforms bytecode to machine code at runtime. The emitted code is specialized for certain data types and is faster than the standard Python code. Psyco does not support Python 2.7 or later. PyS60 was a Python 2 interpreter for Series 60 mobile phones released by Nokia in 2005. It implemented many of the modules from the standard library and some additional modules for integrating with the Symbian operating system. The Nokia N900 also supports Python with GTK widget libraries, enabling programs to be written and run on the target device. Cross-compilers to other languages There are several compilers to high-level object languages, with either unrestricted Python, a restricted subset of Python, or a language similar to Python as the source language: Brython, Transcrypt and Pyjs (latest release in 2012) compile Python to JavaScript. Cython compiles (a superset of) Python to C (while the resulting code is also usable with Python and also e.g. C++). Nuitka compiles Python into C. Numba uses LLVM to compile a subset of Python to machine code. Pythran compiles a subset of Python 3 to C++ (C++11). RPython can be compiled to C, and is used to build the PyPy interpreter of