There is still no complaint about an infix feature with
an invalid name, even if it is declared without any arguments.
Prerelease 92.12.08.

Prerelease 93.02.17: Many invalid feature names are now rejected.  But
some are still accepted, such as
	
	infix "| |" 
	infix "&    " 
	infix "&"  
	infix "not"  

Also, correct number of arguments and result for operator features is
still not enforced.