Add \isdot shim for backwards compatibility with BibTeX

[chbrown]

\isdot is a command provided by BibLaTeX to avoid extra spacing between double punctuation, e.g., when a paper title ends in "et al.". In such cases, you would add \isdot after the initial dot in your .bib file, but if you use BibTeX + LaTeX, the LaTeX compiler chokes with an "Undefined control sequence" error when it hits any \isdot calls.

AFAICT, a simple \newcommand\isdot{} seems to do the trick, but perhaps there are other cases where something fancier might be more appropriate?

The shim could be added to the begin.bib function in sp.bst or to the non-BibLaTeX option loader in sp.cls; I'm not sure which location makes more sense.