ax error should be declared, and recovery
should be made by skipping erroneous tokens (equivalent to matching and 
replacing them with <tt>error</tt>) up to a point at which the parse can 
be continued with a semicolon (and additional context that legally follows a 
statement).  An error is considered to be recovered from if and only if a 
sufficient number of tokens past the <tt>error</tt> symbol can be successfully 
parsed.  (The number of tokens required is determined by the 
<tt>error_sync_size()</tt> m