tuned;

use Content-Digest header in HEAD requests instead of length (to track non-monotone changes);

merged

Simplification of sin, cos, exp of multiples of pi

minor performance tuning: more compact data;

removed unused/inefficient size_of_proof (see also 2241191a3c54);

tuned;

more robust: avoid crash of sleep() for negative time;

clarified signature;

clarified signature: more explicit types;