Function: addprimes
Section: number_theoretical
C-Name: addprimes
Prototype: DG
Help: addprimes({x=[]}): add primes in the vector x to the prime table to
 be used in trial division. x may also be a single integer. Composite
 "primes" are NOT allowed.
Doc: adds the integers contained in the
 vector $x$ (or the single integer $x$) to a special table of
 ``user-defined primes'', and returns that table. Whenever \kbd{factor} is
 subsequently called, it will trial divide by the elements in this table.
 If $x$ is empty or omitted, just returns the current list of extra
 primes.

 The entries in $x$ must be primes: there is no internal check, even if
 the \tet{factor_proven} default is set. To remove primes from the list use
 \kbd{removeprimes}.
