Calculates the derivatives of regular expressions.
% cargo run --example main
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.09s
Running `target/debug/examples/main`
r: (abc)*
d(r, 'a'): bc(abc)*
d(r, 'b'): ∅
d(d(r, 'a'), 'b'): c(abc)*