[BioC] How to get mapping from probe_id to probeset_id in Mouse Exon Array?

[Peng Yu]

MoEx-1_0-st-v1.r2.pgf contains the mapping from probe_id to
probeset_id. But it is not straightforward to be parsed by basic R
functions. Since oligo package processes Exon Array, there must be a
way to get the mapping from oligo package. But I don't find such
information on http://www.bioconductor.org/packages/2.5/bioc/manuals/oligo/man/oligo.pdf.

Could somebody let me know how I should get the mapping information?