A student thesis used for testing of the x-alpha.bst file
showed a problem: because DOI values are now recorded
as

	\href {https://doi.org/10.1234/56789} {10.1234/56789}

instead of the old

	\url{10.1234/56789}

which resulted in an unusable hyperlink, there is a potential new
problem with special characters in DOIs.

Except for use in our archives of %3B to hide semicolons,
I now have no remaining DOI values that require a percent
character.  However, about 3% of 680,000+ recorded DOIs
need the underscore character, which Springer uses to
separate ISBN and chapter data.

I resolved that problem by this definition change:

    \ifx \showDOI \undefined \def \showDOI { \catcode`\_=11 doi:\penalty 0\tt} \fi" write$ newline$

It sets the catcode of underscore to make it a letter.  Without that
change, because \href{}{}'s arguments are passed in math mode, the
underscore disappears, and makes the next character a subscript.

The question is: are there other problem characters that might appear
in DOIs?  I found no instances of caret (^), dollar ($), braces ({}),
or back quote (`) in DOI values in our archives, so I think the above
solution should suffice.

However, the *.ltx files that mention \showDOI should be updated to
note that change.