-for inserting a method for series expansion. Also, the new function
-needs to be declared somewhere. This may also be done by a convenient
-preprocessor macro:
+for inserting a method for series expansion. (If set to @code{NULL} it
+defaults to simple Taylor expansion, which is correct if there are no
+poles involved. The way GiNaC handles poles in case there are any is
+best understood by studying one of the examples, like the Gamma function
+for instance. In essence the function first checks if there is a pole
+at the evaluation point and falls back to Taylor expansion if there
+isn't. Then, the pole is regularized by some suitable transformation.)
+Also, the new function needs to be declared somewhere. This may also be
+done by a convenient preprocessor macro: