A specific request from Joachim Schrod for babel, was the possibility
to extend the definition of a control sequence on the fly. It should,
for instance, be possible that the user adds a macro of his
own to the definition of \extrasenglish.
This feature is now provided by the macro \addto{ 〈:〉!:}control
sequence}{ 〈:〉!:}TEX code}. It is now used
throughout the language-specific files to build the macros
\extras 〈:〉!:}lang and \noextras 〈:〉!:}lang.
The support macros \allowhyphens, \set@low@box and
\save@sf@q have been moved from the language-specific files to
the core of the babel system.