Dual

• Implement standard functions (sin, cos, signum, abs, pow, sqrt)
• Make generic Dual { a: T, b: T }
• Add vector support

I wonder

1. Is there any way to derive commutative operators?
2. How do I make sure d/dx x*y = y?
   • Explicitly tag
   • Generate random numbers
   • Have context maintaing counter?
   • Or just use a vector ...