returns the list of parameters of the ring as a string
or the name of the n-th parameter where n is given by the int_expression.
If the ring_name is omitted, the basering is used, thus
parstr(n) is equivalent to parstr(basering,n).
Example:
ring r=(7,a,b,c),(x,y),wp(2,3);
parstr(r);
==> a,b,c
parstr(2);
==> b
parstr(r,3);
==> c