:: Sets the precision for bigfloat operations to n digits.
return
integer
n
integer
When an argument is given, it
sets the precision for bigfloat operations to n digits.
The return value is always the previous precision in digits regardless of
the existence of an argument.
Bigfloat operations are done by PARI. (See section 6.1.14 pari.)
This is effective for computations in bigfloat.
Refer to ctrl() for turning on the `bigfloat flag.'
There is no upper limit for precision digits.
It sets the precision to some digits around the specified precision.
Therefore, it is safe to specify a larger value.