r,theta
In complex_dilog(), input
values. If theta takes its default value of
NULL, interpret r as a complex-valued object. If
theta is non-null, interpret r as the Modulus,
and theta as the argument, of the complex object passed to
gsl_sf_complex_dilog_e()
give
Boolean, with default FALSE meaning to
return just the answers, and TRUE meaning to return a status
vector as well