the higher precision, the better guess
it can also be an expression, which is evaluated to decimal value serving as a base for the guess; (ha! simple calculator!)
parsing examples: pi/4; 0.(3); 1/7; sin(pi/3); 1/2+1/3+1/4; 0.70710678118655; asin(0.70710678118655); 1.6443229633683; 0.52340425531915
see also function parser, which uses this engine